Reduce the lifetime of open proto index files when building log-to-phys and
phys-to-log indices in FSX.  These files are only required locally, so there is
no reason not to close them as soon as possible.  This changeset fixes the
ENOTEMPTY error when committing things (purging transactions, actually) to FSX
repositories on Windows and unbreaks every non-trivial --fs-type=fsx test.

* subversion/libsvn_fs_x/index.c
  (svn_fs_x__l2p_index_create, svn_fs_x__p2l_index_create):
    For consistency, open the proto index files in LOCAL_POOL.  Explicitly call
    svn_io_file_close() for these files when they are no longer required.

Patch by: Evgeny Kotkov <evgeny.kotkov{_AT_}visualsvn.com>