Do region flush in Region::handleWriteRaftCmd (#542)

[sre-bot]

cherry-pick #542 to release-3.1

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• Move region flush logic into Region::handleWriteRaftCmd, so that it be protected by Region::mutex and Region::predecode_mutex
• Remove remove flushed data logic into RegionTable::writeBlockByRegion from RegionTable:: flushRegion, so that writeBlockByRegion can be called directly from Region::handleWriteRaftCmd. Note that we don't call flushRegion directly because we try to avoid getting instance of RegionTable, some unnecessary check and print log. As Region::handleWriteRaftCmd is very performance critical.

[JaySon-Huang]

/run-integration-tests

[JaySon-Huang]

lgtm