tag: lock all accesses to tag_pool

The tag pool is a shared global resource that is infrequently
modified.  However, it can occasionally be modified by several
threads, especially by the metadata_pipe for streaming metadata
(both reading/writing).

The bulk tag_item pool is NOT locked as currently only the
update thread uses it.

src/tag_pool.c

@@ -19,6 +19,8 @@
 #include "tag_pool.h"
 #include "utils.h"
 
+pthread_mutex_t tag_pool_lock = PTHREAD_MUTEX_INITIALIZER;
+
 #define NUM_SLOTS 4096
 
 struct slot {