Adjust last_ordered_3pc and perform GC when detecting lag in checkpoints on backup

Currently, when a lag in checkpoints is detected on a backup replica, only the watermarks are shifted. This is insufficient to resume ordering of 3PC-batches. To make it possible to resume ordering starting from the 3PC-batch next to the last quorumed checkpoint, last_ordered_3pc must be also adjusted and the messages below its new value should be removed (i.e. garbage collection should be performed). In scope of this task add these actions to the logic of handling a lag in checkpoints on a backup replica.