keeper: add some missing waits for instance ready

When we start an instance during the main logic we should wait that
it's ready (or the next command to update its replication slots will fail).

cmd/keeper/keeper.go

@@ -1349,6 +1349,10 @@ func (p *PostgresKeeper) postgresKeeperSM(pctx context.Context) {
 				log.Errorw("failed to start postgres", zap.Error(err))
 				return
 			}
+			if err = pgm.WaitReady(cluster.DefaultDBWaitReadyTimeout); err != nil {
+				log.Errorw("timeout waiting for instance to be ready", zap.Error(err))
+				return
+			}
 			started = true
 		}