[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

include/rocksdb/perf_context.h

@@ -28,6 +28,8 @@ struct PerfContext {
   uint64_t block_read_time;
   uint64_t block_checksum_time;
   uint64_t block_decompress_time;
+  uint64_t internal_key_skipped_count;
+  uint64_t internal_delete_skipped_count;
 };
 
 extern __thread PerfContext perf_context;