Problem was that temporal buffers allocated for page compression
are not initialized and rest of the page that is actually writen
was als not initialized after previous usage.

Decision about algorihtm moved on prepare phase.
Made possible to add mpore password algorithms.

Change constant with a system variable.

Add support for semi-join strategies: FirstMatch, DuplicateWeedout,
LooseScan.

If we didn't do it, SJ-Materialization table would appear to
EXPLAIN JSON code as having different keyparts than it actually
has. This caused unpredictable content in "used_key_parts"

- Do set nest_level and nest_level_base for fake_select_lex objects.

- Switch Explain data structure from "flat" representation of
  SJ-Materialization into nested one.
- Update functions that print tabular output to operate on the
  nested structure.
- Add function to generate JSON output.

local function variable was overwriting the global one.
also remove unused variable expansion.

control scripts (mysqld_safe for example) want to know
whether a recovery was successful or not.
total silence is too suspicious to be trusted.

MDEV-7188 main.signal_demo3 and sys_vars.max_sp_recursion_depth_func fail in biuldbot on labrador with Thread stack overrun

reduce stack requirememnts for CALL by just a little bit

Added Rows_affected to slow query log & mysql.slow_log table.

(just like in the test below)

Updated MDL_context's lock methods to accept double precision
lock_wait_timeout.

Fixed the condition to make sure mysqld process terminates when
started with wsrep-recover.

- Print r_loops
- Always print r_* members. Print NULL values if no scans took place
- Added testcases.

   Missing GIS functions added:
        IsRing()
        PointOnSurface
        PointOnSurface
        Relate
        Distance
        Intersection
        ConvexHull
   Other old OpenGis standard inconsistencies fixed.

- Basic support for JOIN buffering
- The output is not polished but catches the main point:
  tab->select_cond and tab->cache_select->cond are printed separately.
- Hash join support is poor still.

- Also fixed identation in JOIN_TAB::save_explain_data

MDEV-7228: innodb.innodb-page_compression_tables, innodb.innodb-page_compression_zip, innodb.innodb_stats_create_on_corrupted fail with embedded server

Problem is that tests restart the server and "shutdown_server" looks
for pid file # which is not there with embedded mode.

Fix tests so that they are not run with embedded mode.

Writing JSON:
- Fix a bug in Single_line_formatting_helper
- Add Json_writer_nesting_guard - safety class

EXPLAIN JSON support
- Add basic subquery support
- Add tests for UNION/UNION ALL.

Problem was that test tried to calculate number of pages on
buffer pool and operations done to them. This is not realible
way on buildbot.

Problem is that there is additional error message from function
that is not really needed now.

  Problem is that test could open Microsoft C++ Client Debugger
  windows with abort exceptin. Lets not try to test this on
  windows.