By adding the default value _after_ adding the column we avoid updating
all rows in a table, saving a lot of time and unnecessary work in the
process.

Starting with GitLab 9.1.0 we will no longer allow downtime migrations
unless absolutely necessary. This commit updates the various developer
guides and adds code that is necessary to make zero downtime migrations
less painful.

This reverts commit cb10b725c8929b8b4460f89c9d96c773af39ba6b.

This was initially not implemented simply because I forgot about the
size limit of constraint names in PostgreSQL (63 bytes). Using the old
technique we can't add foreign keys for certain tables. For example,
adding a foreign key on
protected_branch_merge_access_levels.protected_branch_id would lead to
the following key name:

    fk_protected_branch_merge_access_levels_protected_branches_protected_branch_id

This key is 78 bytes long, thus violating the PostgreSQL size
requirements.

The hashing strategy is copied from Rails' foreign_key_name() method,
which unfortunately is private and subject to change without notice.

This method allows one to create foreign keys without blocking access to
the source table, but only on PostgreSQL.

When creating a regular foreign key the "ALTER TABLE" statement used for
this won't return until all data has been validated. This statement in
turn will acquire a lock on the source table. As a result this lock can
be held for quite a long amount of time, depending on the number of rows
and system load.

By breaking up the foreign key creation process in two steps (creation,
and validation) we can reduce the amount of locking to a minimum.
Locking is still necessary for the "ALTER TABLE" statement that adds the
constraint, but this is a fast process and so will only block access for
a few milliseconds.

This reverts commit 13e37a3e.

This ensures that whatever locks are acquired aren't held onto until the
end of the transaction (= after _all_ rows have been updated). Timing
wise there's also no difference between using a transaction and not
using one.

By passing a block to update_column_in_batches() one can now customize
the queries executed. This in turn can be used to only update a specific
set of rows instead of simply all the rows in the table.

This ensures that whenever changing the NULL constraint of a column
fails we still drop the column.

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

These helpers can be used to perform migrations without taking down the
entire application.

For example, the method "add_column_with_default" can be used to add a
new column with a default value without locking the entire table.