Make dss operations lockless.

Rather than protecting dss operations with a mutex, use atomic
operations.  This has negligible impact on synchronization overhead
during typical dss allocation, but is a substantial improvement for
chunk_in_dss() and the newly added chunk_dss_mergeable(), which can be
called multiple times during chunk deallocations.

This change also has the advantage of avoiding tsd in deallocation paths
associated with purging, which resolves potential deadlocks during
thread exit due to attempted tsd resurrection.

This resolves #425.

test/unit/junk.c

@@ -53,10 +53,10 @@ arena_dalloc_junk_large_intercept(void *ptr, size_t usize)
 }
 
 static void
-huge_dalloc_junk_intercept(tsdn_t *tsdn, void *ptr, size_t usize)
+huge_dalloc_junk_intercept(void *ptr, size_t usize)
 {
 
-	huge_dalloc_junk_orig(tsdn, ptr, usize);
+	huge_dalloc_junk_orig(ptr, usize);
 	/*
 	 * The conditions under which junk filling actually occurs are nuanced
 	 * enough that it doesn't make sense to duplicate the decision logic in