[RocksDB] dump leveldb.stats periodically in LOG file.

Summary:
Added an option stats_dump_period_sec to dump leveldb.stats to LOG periodically for diagnosis.
By defauly, it's set to a very big number 3600 (1 hour).

Test Plan: make check;

Reviewers: dhruba

Reviewed By: dhruba

CC: leveldb, zshao

Differential Revision: https://reviews.facebook.net/D10761

db/db_impl.h

@@ -208,6 +208,10 @@ class DBImpl : public DB {
 
 
   Status ReadFirstLine(const std::string& fname, WriteBatch* const batch);
+
+  // dump leveldb.stats to LOG
+  void MaybeDumpStats();
+
   // Constant after construction
   const InternalFilterPolicy internal_filter_policy_;
   bool owns_info_log_;
@@ -272,6 +276,9 @@ class DBImpl : public DB {
   // last time when PurgeObsoleteWALFiles ran.
   uint64_t purge_wal_files_last_run_;
 
+  // last time stats were dumped to LOG
+  uint64_t last_stats_dump_time_microsec_;
+
   // These count the number of microseconds for which MakeRoomForWrite stalls.
   uint64_t stall_level0_slowdown_;
   uint64_t stall_memtable_compaction_;