WL#9699: Change innodb_autoinc_lock_mode default to 2

Affects: Server-8.0   —   Status: Complete   —   Priority: Medium

Since the default replication format is now RBR, InnoDB should also change the 
method of auto increment handling to interleaved.

Upgrade Story
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In previous versions of MySQL, auto-increment allocation was guaranteed to be
sequential in a multi-row insert statement.  This had the effect that the
auto-increment sequence was effectively locked, and parallel multi-row inserts
were not possible.

The guarantee of sequential auto_increment sequences is a requirement of
statement-based replication, but with the default now row-based replication
since MySQL 5.7; it is possible to also change the default auto_increment
behavior to 'interleave' values between statements.

This change is known to be incompatible with statement based replication, and
may break some applications (or user-generated test suites) that depend on a
guaranteed sequential auto_increment.  The previous default can be restored by
setting innodb_autoinc_lock_mode=1.
FR-1) The compiled default value of innodb_autoinc_lock_mode changed from 1
to 2.

NFR-1) Any changes required to MTR tests.