Don't call FindObsoleteFiles() in ~ColumnFamilyHandleImpl() if CF is not dropped (#5238)

Summary:
We have a DB with ~4k column families and ~70k files. On shutdown, destroying the 4k ColumnFamilyHandle-s takes over 2 minutes. Most of this time is spent in VersionSet::AddLiveFiles() called from FindObsoleteFiles() from ~ColumnFamilyHandleImpl(). It's just iterating over the list of files in memory. This seems completely unnecessary as no obsolete files are actually found since the CFs are not even dropped. This PR fixes that.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5238

Differential Revision: D15056342

Pulled By: siying

fbshipit-source-id: 2aa342ef3770b4aa384ce81f8768e485480e4f08

db/column_family.cc

@@ -63,9 +63,14 @@ ColumnFamilyHandleImpl::~ColumnFamilyHandleImpl() {
     JobContext job_context(0);
     mutex_->Lock();
     if (cfd_->Unref()) {
+      bool dropped = cfd_->IsDropped();
+
       delete cfd_;
+
+      if (dropped) {
+        db_->FindObsoleteFiles(&job_context, false, true);
+      }
     }
-    db_->FindObsoleteFiles(&job_context, false, true);
     mutex_->Unlock();
     if (job_context.HaveSomethingToDelete()) {
       bool defer_purge =