XDCR - MapShaRefCounter "needToSync" subjected to race condition

The backend mechanism for keeping track of upserting brokenmapping is subjected to race condition. The flow is as follows:

1. A new instance of mapping is inserted. c.needToSync is set to true
2. {{ MapShaRefCounter}}'s upsertMapping is called
3. It checks the needToSync variable, and unlocks prior to doing RPC to simple store
4. Once RPC returns, it re-acquires mutex to c.needToSync
5. Without ensuring that c.shaToMapping did not change underneath, it clears c.needToSync. It may introduce a situation where a new entry was added to c.shaToMapping but the flag was cleared without actually upserting it.