Sync'ing FSX with FSFS:
Merge r1565100,1590380,1591005,1591018,1591028
from /subversion/libsvn_fs_fs into subversion/libsvn_fs_x.
Conflicts were due naming differences between FSX and FSFS.

This ports the fulltext handling (memory and caching) patches.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1595593 13f79535-47bb-0310-9956-ffa450edef68