Sync FSX with FSFS:
Merge and adapt revisions r1604188, r1604413, r1604442, r1604700, r1604726
r1604755 and r1604844 from libsvn_fs_fs.  Various conflicts were caused by
reordered code and renamed functions in FSX.

This ports memory / pool usage improvements in the parser and lower data
access layers.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1635101 13f79535-47bb-0310-9956-ffa450edef68