Check region epoch before updating the region approximate size

[niebayes]

When a peer receives a MsgTypeRegionApproximateSize message from the split checker, it simply updates its cached ApproximateSize without any checking.
The region might be recently split when the peer receives this message. On this occasion, the update seems wrong.
I wonder if the checking for region epoch is necessary before the update.

[Connor1996]

As the messages of one peer are serialized in one channel, the MsgTypeRegionApproximateSize should be handled before split is finished