Sync'ing FSX with FSFS:
Merge r1533396,1544717,1544719,1544721,1559777,1590385,1590416,1590440
from /subversion/libsvn_fs_fs into subversion/libsvn_fs_x.  Conflicts
were due earlier cherry-picks and structural differences between FSX
and FSFS.

This ports a number of local improvements.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1595309 13f79535-47bb-0310-9956-ffa450edef68