Sync FSX with FSFS:
Merge revisions r1604802, r1604824, r1604836, r1604902, r1604911,
r1604925, r1605060, r1605065, r1605068, r1605071, r1605072 and r1605073
from libsvn_fs_fs.  Fix the usual text conflicts caused by reordered code
and renamed functions in FSX.

This ports dual pool paradigm introduction in the lower data access layer
to FSX.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1635377 13f79535-47bb-0310-9956-ffa450edef68