Sync FSX with FSFS:
Merge revisions r1612810, r1613872, r1616338, r1616339, r1617688, r1618138,
r1628738, r1629857 and r1629865 from libsvn_fs_fs and resolve the usual FSX
conflicts.

This ports the latest hotcopy improvements to FSX.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1635612 13f79535-47bb-0310-9956-ffa450edef68