Add rate_delay_limit_milliseconds

Summary:
This adds the rate_delay_limit_milliseconds option to make the delay
configurable in MakeRoomForWrite when the max compaction score is too high.
This delay is called the Ln slowdown. This change also counts the Ln slowdown
per level to make it possible to see where the stalls occur.

From IO-bound performance testing, the Level N stalls occur:
* with compression -> at the largest uncompressed level. This makes sense
                      because compaction for compressed levels is much
                      slower. When Lx is uncompressed and Lx+1 is compressed
                      then files pile up at Lx because the (Lx,Lx+1)->Lx+1
                      compaction process is the first to be slowed by
                      compression.
* without compression -> at level 1

Task ID: #1832108

Blame Rev:

Test Plan:
run with real data, added test

Revert Plan:

Database Impact:

Memcache Impact:

Other Notes:

EImportant:

- begin *PUBLIC* platform impact section -
Bugzilla: #
- end platform impact -

Reviewers: dhruba

Reviewed By: dhruba

Differential Revision: https://reviews.facebook.net/D9045

include/leveldb/options.h

@@ -324,6 +324,9 @@ struct Options {
   // exceeds rate_limit. This is ignored when <= 1.0.
   double rate_limit;
 
+  // Max time a put will be stalled when rate_limit is enforced
+  int rate_limit_delay_milliseconds;
+
   // manifest file is rolled over on reaching this limit.
   // The older manifest file be deleted.
   // The default value is MAX_INT so that roll-over does not take place.