WL#9631: Change InnoDB IO Defaults - innodb_flush_neighbors

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

Since fast IO (SSDs) is now the default for deployment, we should flip the
default of flush_neighbors to be disabled.  This disables merging which may have
a negative impact on performance.

In summary:
innodb_flush_neighbors=0

Concerns/Investigation
----------------------
SYS-QA will verify that disabling innodb_flush_neighbors by default, doesn't
have performance impact. 

Upgrade Story
-------------
This new default changes MySQL to cater for SSDs and fast storage devices by
default.  We expect that for the majority of users, this will result in a small
performance gain.  Users who are using slower hard drives may see a performance
loss, and are encouraged to revert to the previous defaults by setting
innodb_flush_neighbors=1.
FR-1) The compiled default value of innodb_flush_neighbors changed from 1
to 0.

NFR-1) Any changes required to MTR tests.

NFR1) Performance should actually be better on fast IO systems with
flush-neighbors.  It is expected that performance may drop on slow IO systems.