Sync'ing FSX with FSFS:
Merge r1584235,1590460,1590464 from /subversion/libsvn_fs_fs into
subversion/libsvn_fs_x.  Conflicts were due structural and naming
differences between FSX and FSFS.

This ports the "independent p2l index page size" patches.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1595582 13f79535-47bb-0310-9956-ffa450edef68