Deallocate future mutex in the future resource destructor

Otherwise we end up leaking mutexes https://github.com/apache/couchdb-erlfdb/issues/49
diff --git a/c_src/resources.c b/c_src/resources.c
index 88f402a..9be74af 100644
--- a/c_src/resources.c
+++ b/c_src/resources.c
@@ -74,6 +74,10 @@
     if(f->msg_env != NULL) {
         enif_free_env(f->msg_env);
     }
+
+    if(f->lock != NULL) {
+        enif_mutex_destroy(f->lock);
+    }
 }