PurgeObsoleteFiles() unittest

Summary:
Created a unittest that verifies that automatic deletion performed by PurgeObsoleteFiles() works correctly.

Also, few small fixes on the logic part -- call version_set_->GetObsoleteFiles() in FindObsoleteFiles() instead of on some arbitrary positions.

Test Plan: Created a unit test

Reviewers: dhruba, haobo, nkg-

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14079

include/rocksdb/options.h

@@ -387,7 +387,9 @@ struct Options {
   bool disable_seek_compaction;
 
   // The periodicity when obsolete files get deleted. The default
-  // value is 6 hours.
+  // value is 6 hours. The files that get out of scope by compaction
+  // process will still get automatically delete on every compaction,
+  // regardless of this setting
   uint64_t delete_obsolete_files_period_micros;
 
   // Maximum number of concurrent background jobs, submitted to