Sync'ing FSX with FSFS:
Merge r1554942,1555297,1555340,1575453,1575622,1580406,1580623,1580629,
1590383 from /subversion/libsvn_fs_fs into subversion/libsvn_fs_x.
Conflicts were due structural and naming differences between FSX and FSFS.

This ports the delta window caching and usage patches.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1595589 13f79535-47bb-0310-9956-ffa450edef68