LoadRegion from persistent storage should remove overlap region after compare version

[disksing]

Currently, when PD loads regions from persistent storage, it inserts regions to the region tree one by one. When there are regions have overlapped range, the region that loaded earlier will be removed from the tree. See https://github.com/pingcap/pd/blob/035f65eebbd028449d9ebb06cb166ab3c266136d/server/cluster/cluster.go#L245

Because this process does not involve any epoch comparison, it may lead to gaps in the result region tree. For example, region A [m, x) merged to region B [c, m), produces new region B [c, x). If region B has a smaller region ID, it will be loaded before A and be removed when A is loaded, resulting a gap [c, m).

Possible fix is to compare region version and remove the one with older epoch.

[disksing]

#3808 fixes this issue more completely. Regions are deleted at the follower after they are merged, rather than waiting until the reboot loading phase.