The deletion of obsolete files should not occur very frequently.

Summary:
The method DeleteObsolete files is a very costly methind, especially
when the number of files in a system is large. It makes a list of
all live-files and then scans the directory to compute the diff.
By default, this method is executed after every compaction run.

This patch makes it such that DeleteObsolete files is never
invoked twice within a configured period.

Test Plan: run all unit tests

Reviewers: heyongqiang, MarkCallaghan

Reviewed By: MarkCallaghan

Differential Revision: https://reviews.facebook.net/D6045

db/db_impl.h

@@ -197,6 +197,9 @@ class DBImpl : public DB {
   // shall we disable deletion of obsolete files
   bool disable_delete_obsolete_files_;
 
+  // last time when DeleteObsoleteFiles was invoked
+  uint64_t delete_obsolete_files_last_run_;
+
   // Per level compaction stats.  stats_[level] stores the stats for
   // compactions that produced data for the specified "level".
   struct CompactionStats {