Added more logging in HA

A slave will not go from SLAVE to PENDING if the newly selected master is the same as the old one.
This helps to prevent possible race during new instance joining the cluster and getting an available slave.
Because after we've introduced UpdatePuller, the catch-up is done by UpdatePuller periodically. The test should go to TestPullUpdates, and we've had a very similar test "TestPullUpdates#makeSureUpdatePullerGetsGoingAfterMasterSwitch". Besides, the original goal of this test is to verify unpacker catches up first when applying tx on slave. This behaviour is no longer the same after we've had UpdatePuller.