add io mutex to cache db for optional mutex protection

git-svn-id: http://svn.freeswitch.org/svn/freeswitch/trunk@15800 d0543943-73ff-0310-b7d9-9358b9ac24b2

src/include/switch_core.h

@@ -1987,6 +1987,7 @@ typedef struct {
 	switch_cache_db_native_handle_t native_handle;
 	time_t last_used;
 	switch_mutex_t *mutex;
+	switch_mutex_t *io_mutex;
 	switch_memory_pool_t *pool;
 	int32_t flags;
 	unsigned long hash;