Sync FSX with FSFS:  Block (record-only merge) revisions
r1604933, r1606599, r1606656, r1606658, r1606662 and r1629873
from libsvn_fs_fs.

These made "block-read" and optional feature.  It stays mandatory
in FSX, though.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1635972 13f79535-47bb-0310-9956-ffa450edef68