Allow users to profile a query and see bottleneck of the query

Summary:
Provide a framework to profile a query in detail to figure out latency bottleneck. Currently, in Get(), Put() and iterators, 2-3 simple timing is used. We can easily add more profile counters to the framework later.

Test Plan: Enable this profiling in seveal existing tests.

Reviewers: haobo, dhruba, kailiu, emayanke, vamsi, igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14001

Conflicts:
	table/merger.cc

include/rocksdb/perf_context.h

@@ -38,7 +38,27 @@ struct PerfContext {
   uint64_t internal_key_skipped_count;
   // total number of deletes skipped over during iteration
   uint64_t internal_delete_skipped_count;
-  uint64_t wal_write_time;            // total time spent on writing to WAL
+
+  uint64_t get_snapshot_time;          // total time spent on getting snapshot
+  uint64_t get_from_memtable_time;     // total time spent on querying memtables
+  uint64_t get_from_memtable_count;    // number of mem tables queried
+  // total time spent after Get() finds a key
+  uint64_t get_post_process_time;
+  uint64_t get_from_output_files_time; // total time reading from output files
+  // total time spent on seeking child iters
+  uint64_t seek_child_seek_time;
+  // number of seek issued in child iterators
+  uint64_t seek_child_seek_count;
+  uint64_t seek_min_heap_time;         // total time spent on the merge heap
+  // total time spent on seeking the internal entries
+  uint64_t seek_internal_seek_time;
+  // total time spent on iterating internal entries to find the next user entry
+  uint64_t find_next_user_entry_time;
+  // total time spent on pre or post processing when writing a record
+  uint64_t write_pre_and_post_process_time;
+  uint64_t write_wal_time;            // total time spent on writing to WAL
+  // total time spent on writing to mem tables
+  uint64_t write_memtable_time;
 };
 
 extern __thread PerfContext perf_context;