Add -enable_pipelined_write to db_bench and add two defaults

Summary:
Expose pipeline write in db_bench and change the default to parallel memtable inserts
Closes https://github.com/facebook/rocksdb/pull/2527

Differential Revision: D5359825

Pulled By: siying

fbshipit-source-id: e30755feb07ff19a731c4058acf101e02de4e197

tools/db_bench_tool.cc

@@ -776,10 +776,13 @@ DEFINE_uint64(delayed_write_rate, 8388608u,
               "Limited bytes allowed to DB when soft_rate_limit or "
               "level0_slowdown_writes_trigger triggers");
 
-DEFINE_bool(allow_concurrent_memtable_write, false,
+DEFINE_bool(enable_pipelined_write, true,
+            "Allow WAL and memtable writes to be pipelined");
+
+DEFINE_bool(allow_concurrent_memtable_write, true,
             "Allow multi-writers to update mem tables in parallel.");
 
-DEFINE_bool(enable_write_thread_adaptive_yield, false,
+DEFINE_bool(enable_write_thread_adaptive_yield, true,
             "Use a yielding spin loop for brief writer thread waits.");
 
 DEFINE_uint64(
@@ -3105,6 +3108,7 @@ void VerifyDBFromDB(std::string& truth_db_name) {
         FLAGS_allow_concurrent_memtable_write;
     options.enable_write_thread_adaptive_yield =
         FLAGS_enable_write_thread_adaptive_yield;
+    options.enable_pipelined_write = FLAGS_enable_pipelined_write;
     options.write_thread_max_yield_usec = FLAGS_write_thread_max_yield_usec;
     options.write_thread_slow_yield_usec = FLAGS_write_thread_slow_yield_usec;
     options.rate_limit_delay_max_milliseconds =