[RocksDB] Added perf counters to track skipped internal keys during iteration

Summary: as title. unit test not polished. this is for a quick live test

Test Plan: live

Reviewers: dhruba

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13221

util/perf_context.cc

@@ -15,6 +15,8 @@ void PerfContext::Reset() {
   block_read_time = 0;
   block_checksum_time = 0;
   block_decompress_time = 0;
+  internal_key_skipped_count = 0;
+  internal_delete_skipped_count = 0;
 }
 
 __thread PerfContext perf_context;