Apply a recent FSFS optimisation (r1569549) to FSX.

The txn-list-lock protects the shared txn structures but not the
protorev file so the txn-list-lock can be released before doing the
disk IO to open, and seek to the end of, the protorev file.  This
reduces contention for the txn-list-lock.

* subversion/libsvn_fs_x/transaction.c
  (unlock_proto_rev_list_locked): Remove.
  (struct get_writable_proto_rev_baton): Remove file member.
  (get_writable_proto_rev_body): Move open/seek from here ...
  (get_writable_proto_rev): ... to here.



git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1571676 13f79535-47bb-0310-9956-ffa450edef68