Fix an initialization / life time issue with FSX's batch-fsync.
The svn_fs_x__init function may be called more than once and the FSX module might later be unloaded and reloaded. So, make sure to create the thread pool only once and to clean it up when the module unloads.
* subversion/libsvn_fs_x/batch_fsync.h (svn_fs_x__batch_fsync_init): Add a owner pool parameter.
* subversion/libsvn_fs_x/batch_fsync.c (thread_pool_initialized): Init flag to be used with svn_atomic__init_once. (thread_pool_pre_cleanup): Cleanup only once but also make the batch- fsync API re-initializable. (create_thread_pool): Move the init code to here. Destroy the thread pool whenever either the owning pool (i.e. during unload) or the containing pool (apr_terminate) got cleaned up. (svn_fs_x__batch_fsync_init): Run the actual initialization only once. (svn_fs_x__batch_fsync_run): Error out on cases where the FS API init / tear-down was not properly ordered with the API usage.