Sync FSX with FSFS:  Merge revisions
r1626871, r1626873, r1626886, r1627497 and r1627498 from libsvn_fs_fs
and resolve the usual text conflicts with FSX.

These patches switch the index.* code to using the two-pool paradigm.
As part of the conflict resolution, some of the value checks in the
index parser have been ported / kept from the conflict as well.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1635862 13f79535-47bb-0310-9956-ffa450edef68