[Rocksdb] Submit mem table flush job in a different thread pool

Summary: As title. This is just a quick hack and not ready for commit. fails a lot of unit test. I will test/debug it directly in ViewState shadow .

Test Plan: Try it in shadow test.

Reviewers: dhruba, xjin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D12933

util/options.cc

@@ -54,6 +54,7 @@ Options::Options()
       disable_seek_compaction(false),
       delete_obsolete_files_period_micros(0),
       max_background_compactions(1),
+      max_background_flushes(0),
       max_log_file_size(0),
       log_file_time_to_roll(0),
       keep_log_file_num(1000),
@@ -199,6 +200,8 @@ Options::Dump(Logger* log) const
         delete_obsolete_files_period_micros);
     Log(log,"             Options.max_background_compactions: %d",
         max_background_compactions);
+    Log(log,"                 Options.max_background_flushes: %d",
+        max_background_flushes);
     Log(log,"                      Options.soft_rate_limit: %.2f",
         soft_rate_limit);
     Log(log,"                      Options.hard_rate_limit: %.2f",