[RocksDB] make SetPerfLevel affect only the current thread

Summary: as title, make it easy to turn on/off profiling at per thread level.

Test Plan: make check

Reviewers: sdong, ljin

Reviewed By: ljin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17469

util/perf_context.cc

@@ -10,7 +10,7 @@
 namespace rocksdb {
 
 // by default, enable counts only
-PerfLevel perf_level = kEnableCount;
+__thread PerfLevel perf_level = kEnableCount;
 
 void SetPerfLevel(PerfLevel level) { perf_level = level; }
 

util/perf_context_imp.h

@@ -9,7 +9,7 @@
 
 namespace rocksdb {
 
-extern enum PerfLevel perf_level;
+extern __thread PerfLevel perf_level;
 
 inline void StartPerfTimer(StopWatchNano* timer) {
   if (perf_level >= PerfLevel::kEnableTime) {