Sync FSX with FSFS:
Merge and adapt revisions r1604416, r1604417 and 1604421 from libsvn_fs_fs.
The usual conflicts caused by reordered code and renamed functions in FSX
had to be resolved.

This ports the "indexes as part of the rev / pack file" changes to FSX.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1635346 13f79535-47bb-0310-9956-ffa450edef68