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