cache SuperVersion in thread local storage to avoid mutex lock

Summary: as title

Test Plan:
asan_check
will post results later

Reviewers: haobo, igor, dhruba, sdong

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16257

include/rocksdb/options.h

@@ -714,6 +714,10 @@ struct Options {
   //
   // Default: 0 (disabled)
   size_t max_successive_merges;
+
+  // Allow RocksDB to use thread local storage to optimize performance.
+  // Default: true
+  bool allow_thread_local;
 };
 
 //

include/rocksdb/statistics.h

@@ -122,6 +122,7 @@ enum Tickers {
   // Number of table's properties loaded directly from file, without creating
   // table reader object.
   NUMBER_DIRECT_LOAD_TABLE_PROPERTIES,
+  NUMBER_SUPERVERSION_UPDATES,
   TICKER_ENUM_MAX
 };
 
@@ -176,7 +177,9 @@ const std::vector<std::pair<Tickers, std::string>> TickersNameMap = {
     {COMPACT_READ_BYTES, "rocksdb.compact.read.bytes"},
     {COMPACT_WRITE_BYTES, "rocksdb.compact.write.bytes"},
     {NUMBER_DIRECT_LOAD_TABLE_PROPERTIES,
-     "rocksdb.number.direct.load.table.properties"}, };
+     "rocksdb.number.direct.load.table.properties"},
+    {NUMBER_SUPERVERSION_UPDATES, "rocksdb.number.superversion_updates"},
+};
 
 /**
  * Keep adding histogram's here.