[Rocksdb] Log on disable/enable file deletions

Summary: as title

Test Plan: compile

Reviewers: dhruba, haobo

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D11085

db/db_filesnapshot.cc

@@ -17,12 +17,14 @@ namespace leveldb {
 Status DBImpl::DisableFileDeletions() {
   MutexLock l(&mutex_);
   disable_delete_obsolete_files_ = true;
+  Log(options_.info_log, "File Deletions Disabled");
   return Status::OK();
 }
 
 Status DBImpl::EnableFileDeletions() {
   MutexLock l(&mutex_);
   disable_delete_obsolete_files_ = false;
+  Log(options_.info_log, "File Deletions Enabled");
   return Status::OK();
 }