MB-50034: Potential deadlock in MOI.doPersistSnapshot

RWMutex RUnlock call is missing. This is a regression introduced with MB-46507 [Backport] - [System test Upgrade] - Negative values returned for stat raw_data_size post upgrade
Due to missing unlock index main loop will get stuck and cause multiple issues such as failed rebalance, indexer flush stuck etc.
Fix: release acquired mutex.

Change-Id: I3c934654e16d874a46fe536a507034bae27f6545

secondary/indexer/memdb_slice_impl.go

@@ -963,6 +963,7 @@ func (mdb *memdbSlice) doPersistSnapshot(s *memdbSnapshot) {
 			os.RemoveAll(tmpdir)
 			mdb.confLock.RLock()
 			maxThreads := mdb.sysconf["settings.moi.persistence_threads"].Int()
+			mdb.confLock.RUnlock()
 			total := atomic.LoadInt64(&totalMemDBItems)
 			indexCount := mdb.GetCommittedCount()
 			// Compute number of workers to be used for taking backup