HBASE-25506 ServerManager#startChore affects MTTR of HMaster (#2889)

Signed-off-by: Duo Zhang <zhangduo@apache.org>

Author: cuibo01

hbase-server/src/main/java/org/apache/hadoop/hbase/master/ServerManager.java

@@ -921,8 +921,13 @@ public boolean isClusterShutdown() {
   public void startChore() {
     Configuration c = master.getConfiguration();
     if (persistFlushedSequenceId) {
-      // when reach here, RegionStates should loaded, firstly, we call remove deleted regions
-      removeDeletedRegionFromLoadedFlushedSequenceIds();
+      new Thread(() -> {
+        // after AM#loadMeta, RegionStates should be loaded, and some regions are
+        // deleted by drop/split/merge during removeDeletedRegionFromLoadedFlushedSequenceIds,
+        // but these deleted regions are not added back to RegionStates,
+        // so we can safely remove deleted regions.
+        removeDeletedRegionFromLoadedFlushedSequenceIds();
+      }, "RemoveDeletedRegionSyncThread").start();
       int flushPeriod = c.getInt(FLUSHEDSEQUENCEID_FLUSHER_INTERVAL,
           FLUSHEDSEQUENCEID_FLUSHER_INTERVAL_DEFAULT);
       flushedSeqIdFlusher = new FlushedSequenceIdFlusher(