Sync FSX with FSFS:
Merge revisions r1604947, r1605064, r1605188, r1605189 and r1605191 from
libsvn_fs_fs and resolve the usual FSX conflicts.

This ports various 'svn log' memory consumption improvements to FSX.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1635497 13f79535-47bb-0310-9956-ffa450edef68