Reduce performance logging to DEBUG (#6833)

* Reduce tnx performance logging to DEBUG
* Changelog.d

changelog.d/6833.misc

@@ -0,0 +1 @@
+Reducing log level to DEBUG for synapse.storage.TIME.

synapse/storage/database.py

@@ -343,7 +343,7 @@ class Database(object):
 
             top_three_counters = self._txn_perf_counters.interval(duration, limit=3)
 
-            perf_logger.info(
+            perf_logger.debug(
                 "Total database time: %.3f%% {%s}", ratio * 100, top_three_counters
             )