Step#2:

1. Removes the function build_equal_items_for_cond() and
   introduces a new method Item::build_equal_items() instead,
   with specific implementations in the following Items:

   Item  (the default implementation)
   Item_ident_or_func_or_sum
   Item_cond
   Item_cond_and

2. Adds a new abstract class Item_ident_or_func_or_sum,
   a common parent for Item_ident and Item_func_or_sum,
   as they have exactly the same build_equal_items().

3. Renames Item_cond_and::cond_equal to Item_cond_and::m_cond_equal,
   to avoid confusion between the member and local variables named
   "cond_equal".

to constructor Create_field::Create_field().

TODO: move some methods from Item to Type_std_attributes.

between Item_func, Item_sum, Item_row.

MDEV-8031: Parallel replication stops on "connection killed" error (probably incorrectly handled deadlock kill)

There was a rare race, where a deadlock error might not be correctly
handled, causing the slave to stop with something like this in the error
log:

150423 14:04:10 [ERROR] Slave SQL: Connection was killed, Gtid 0-1-2, Internal MariaDB error code: 1927
150423 14:04:10 [Warning] Slave: Connection was killed Error_code: 1927
150423 14:04:10 [Warning] Slave: Deadlock found when trying to get lock; try restarting transaction Error_code: 1213
150423 14:04:10 [Warning] Slave: Connection was killed Error_code: 1927
150423 14:04:10 [Warning] Slave: Connection was killed Error_code: 1927
150423 14:04:10 [ERROR] Error running query, slave SQL thread aborted. Fix the problem, and restart the slave SQL thread with "SLAVE START". We stopped at log 'master-bin.000001 position 1234

The problem was incorrect error handling. When a deadlock is detected, it
causes a KILL CONNECTION on the offending thread. This error is then later
converted to a deadlock error, and the transaction is retried.

However, the deadlock error was not cleared at the start of the retry, nor
was the lingering kill signal. So it was possible to get another deadlock
kill early during retry. If this happened with particular thread
scheduling/timing, it was possible that the new KILL CONNECTION error was
masked by the earlier deadlock error, so that the second kill was not
properly converted into a deadlock error and retry.

This patch adds code that clears the old error and killed flag before
starting the retry. It also adds code to handle a deadlock kill caught in a
couple of places where it was not handled before.

Fix a silly typo that caused the test to occasionally fail.

This was a regression from the patch for MDEV-7668.

A test was incorrect, so the slave would not properly handle re-using
temporary tables, which lead to replication failure in this case.

Conflicts:
	mysql-test/suite/multi_source/multisource.result
	sql/sql_base.cc

Removing "Field *result_field" from Item_field and Item_ref,
and deriving Item_ident and Item_ref from Item_result_field
instead.

as the other decendants of Item_result_field (Item_avg_field,
Item_variance_field, Item_cache_wrapper) don't need fix_length_and_dec().

- Adding a new class Item_args, represending regular function or
  aggregate function arguments array.

- Adding a new class Item_func_or_sum,
  a parent class for Item_func and Item_sum

- Moving Item_result_field::name() to Item_func_or_sum(),
  as name() is not needed on Item_result_field level.

Post-fix : A typo and better error handling.

Add some suppressions that were missing. They are for if a STOP SLAVE is
executed early during IO thread startup, when it is negotiating with the
master. The master connection may be killed in the middle of a
mysql_real_query(), which is not a test failure if it is a network error.

This also caught one real code error, fixed with this commit: The I/O thread
would fail to automatically reconnect if a network error happened while
fetching the value of @@GLOBAL.gtid_domain_id.

MDEV-7936: Assertion `!table || table->in_use == _current_thd()' failed on parallel replication in optimistic mode

Additional 10.1-specific test case.

Conflicts:
	sql/sql_base.cc

MDEV-7936: Assertion `!table || table->in_use == _current_thd()' failed on parallel replication in optimistic mode

Make sure that in parallel replication, we execute wait_for_prior_commit()
before setting table->in_use for a temporary table. Otherwise we can end up
with two parallel replication worker threads competing with each other for
use of a temporary table.

Re-factor the use of find_temporary_table() to be able to handle errors
in the caller (as wait_for_prior_commit() can return error in case of
deadlock kill).

MDEV-7668: Intermediate master groups CREATE TEMPORARY with INSERT, causing parallel replication failure

[This commit cherry-picked to be able to merge MDEV-7936, of which it
is a pre-requisite, into both 10.0 and 10.1.]

Parallel replication depends on locking (table locks, row locks, etc.) to
prevent two conflicting transactions from running and committing in parallel.
But temporary tables are designed to be visible only to one thread, and have
no such locking.

In the concrete issue, an intermediate master could commit a CREATE TEMPORARY
TABLE in the same group commit as in INSERT into that table. Thus, a
lower-level master could attempt to run them in parallel and get an error.

More generally, we need protection from parallel replication trying to run
transactions in parallel that access a common temporary table.

This patch simply causes use of a temporary table from parallel replication
to wait for all previous transactions to commit, serialising the replication
at that point.

(A more fine-grained locking could be added later, possibly. However,
using temporary tables in statement-based replication is in any case
normally undesirable; for example a restart of the server will lose
temporary tables and can break replication).

Note that row-based replication is not affected, as it does not do any
temporary tables on the slave-side.

This patch also cleans up the locking around protecting the list of
temporary tables in Relay_log_info. This used to take the
rli->data_lock at the end of every statement, which is very bad for
concurrency. With this patch, the lock is not taken unless temporary
tables (with statement-based binlogging) are in use on the slave.

Fix EXPLAIN FORMAT=JSON to produce output that's not worse than
the tabular form.