Sync FSX with FSFS:
Merge revisions r1606514, r1606526, r1606528, r1607572 and r1623007
from libsvn_fs_fs, resolve the usual FSX conflicts and adapt the code
to FSX.

This port adds the "index (re-)creation utilities" to FSX.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1635528 13f79535-47bb-0310-9956-ffa450edef68