Sync FSX with FSFS:
Merge revisions r1615351 and r1623379 from libsvn_fs_fs and resolve the
usual FSX ordering and naming conflicts.

This ports the latest locking improvements to FSX.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1635614 13f79535-47bb-0310-9956-ffa450edef68