BUG/MINOR: 51d: Releases workset back to pool.

The workset is now released correctly when a cache hit occurs.

This should be backported to 1.6.
diff --git a/src/51d.c b/src/51d.c
index 9bba94e..0211dd7 100644
--- a/src/51d.c
+++ b/src/51d.c
@@ -384,6 +384,7 @@
 		lru = lru64_get(_51d_req_hash(args, ws),
 		                _51d_lru_tree, (void*)args, 0);
 		if (lru && lru->domain) {
+			fiftyoneDegreesWorksetPoolRelease(global._51degrees.pool, ws);
 			_51d_retrieve_cache_entry(smp, lru);
 			return 1;
 		}