FSX implementation of FSFS r1549907.

Make sure that the temporary transaction properties associated with
commit flags do not disappear when svn_fs_commit_txn is interrupted.
As a side effect this also reduces by one the number of times the
proto-revprop file is written in a typical commit.

* subversion/libsvn_fs_fs/structure: Describe new file.

* subversion/libsvn_fs_fs/fs.h
  (PATH_TXN_PROPS_FINAL): New.

* subversion/libsvn_fs_fs/transaction.c
  (svn_fs_x__change_txn_props): Rename to ...
  (change_txn_props): ... this, add boolean parameter.
  (svn_fs_x__change_txn_props): New wrapper.
  (write_final_revprop): New.
  (commit_body): Call write_final_revprop.

* subversion/libsvn_fs_fs/util.h
  (svn_fs_x__path_txn_props_final): New.

* subversion/libsvn_fs_fs/util.c
  (svn_fs_x__path_txn_props_final): New.
  (set_txn_proplist): Add boolean final flag.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1550164 13f79535-47bb-0310-9956-ffa450edef68