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

include/leveldb/options.h

@@ -240,6 +240,11 @@ struct Options {
   // (which is true if max_open_files is large).
   bool disable_seek_compaction;
 
+  // The periodicity when obsolete files get deleted. The default
+  // value is 0 which means that obsolete files get removed after
+  // every compaction run.
+  uint64_t delete_obsolete_files_period_micros;
+
   // Create an Options object with default values for all fields.
   Options();