Sync FSX with FSFS:
Merge revisions r1606132 from libsvn_fs_fs, adapt the code and resolve
the usual FSX conflicts.

This switches the svn_fs_x__revision_file API to the two-pool paradigm.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1635509 13f79535-47bb-0310-9956-ffa450edef68