tikv: cleanup outdate regions's memory from region-cache in 2.1

[lysu]

What problem does this PR solve?

current region-cache should not clean up regions out of TTL #4506

this takes the risk to leave many invalid regions in the region-cache, although LocateKey and getCachedRegion can be free lock, it will hurt the DropStoreOnFail's performance(hold longer lock), for a long-running cluster with many regions, send tikv fail will be slow.

What is changed and how it works?

to control the modification, just add clear up logic in DropStoreOnFail(they already hold the lock to loops all regions, so this will not introduce any other locks cost)

Check List

Tests

• Unit test
• Integration test

Code changes

• impl change

Side effects

• n/a

Related changes

• n/a

Release note

• cleanup outdate regions's memory from region-cache

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This change is 

[coocood]

LGTM

[nolouch]

The long-time is spent in locateing key? maybe we can also update the btree version to the last, there is an improvement in seek.

[jackysp]

LGTM

[jackysp]

Should we update btree in master and release-3.0?

[sre-bot]

/run-all-tests

[sre-bot]

/run-all-tests