[RocksDB] Add perf_context.wal_write_time to track time spent on writing the recovery log.

Summary: as title

Test Plan: make check; ./perf_context_test

Reviewers: dhruba

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13629

util/perf_context.cc

@@ -22,6 +22,7 @@ void PerfContext::Reset() {
   block_decompress_time = 0;
   internal_key_skipped_count = 0;
   internal_delete_skipped_count = 0;
+  wal_write_time = 0;
 }
 
 __thread PerfContext perf_context;