Sync FSX with FSFS:
Merge revisions r1605075 and r1605197 and r1606551 from libsvn_fs_fs
and resolve the usual FSX conflicts.

This ports pool usage improvements to FSX.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1635498 13f79535-47bb-0310-9956-ffa450edef68