Backport fsync queue compaction logic to all supported branches.

Backport fsync queue compaction logic to all supported branches.

This backports commit 7f242d880b5b5d9642675517466d31373961cf98,
except for the counter in pg_stat_bgwriter.  The underlying problem
(namely, that a full fsync request queue causes terrible checkpoint
behavior) continues to be reported in the wild, and this code seems
to be safe and robust enough to risk back-porting the fix.