fix: revert chain sync mutex to a regular lock

Conditions always call "unlock" so we can't safely use the condition
with both the read and write side of lock. So we might as well revert
back to a regular lock.

fixes #10616

blockstore/splitstore/splitstore.go

@@ -188,7 +188,7 @@ type SplitStore struct {
 	cancel func()
 
 	outOfSync         int32 // for fast checking
-	chainSyncMx       sync.RWMutex
+	chainSyncMx       sync.Mutex
 	chainSyncCond     sync.Cond
 	chainSyncFinished bool // protected by chainSyncMx
 

blockstore/splitstore/splitstore_compact.go

@@ -917,8 +917,8 @@ func (s *SplitStore) waitForSync() {
 	if atomic.LoadInt32(&s.outOfSync) == 0 {
 		return
 	}
-	s.chainSyncMx.RLock()
-	defer s.chainSyncMx.RUnlock()
+	s.chainSyncMx.Lock()
+	defer s.chainSyncMx.Unlock()
 
 	for !s.chainSyncFinished {
 		s.chainSyncCond.Wait()