refactor: get rid of dbQueryTimings from SchemaCache

Schema cache query timings are only needed immediately after a schema-cache reload to emit SchemaCacheQueriedObs. Storing them inside SchemaCache makes the cache carry transient observability data that is not part of the cached schema state and is never used by request handling.

This change makes querySchemaCache to return query timings in a tuple in parallel to SchemaCache and removes dbQueryTimings field.

test/observability/Main.hs

@@ -71,4 +71,4 @@ main = do
 
   where
     loadSCache pool conf =
-      either (panic.show) id <$> P.use pool (HT.transaction HT.ReadCommitted HT.Read $ querySchemaCache conf)
+      either (panic.show) fst <$> P.use pool (HT.transaction HT.ReadCommitted HT.Read $ querySchemaCache conf)

test/spec/Main.hs

@@ -289,4 +289,4 @@ main = do
 
   where
     loadSCache pool conf =
-      either (panic.show) id <$> P.use pool (HT.transaction HT.ReadCommitted HT.Read $ querySchemaCache conf)
+      either (panic.show) fst <$> P.use pool (HT.transaction HT.ReadCommitted HT.Read $ querySchemaCache conf)