Improve RocksDB "get" performance by computing merge result in memtable

Summary:
Added an option (max_successive_merges) that can be used to specify the
maximum number of successive merge operations on a key in the memtable.
This can be used to improve performance of the "get" operation. If many
successive merge operations are performed on a key, the performance of "get"
operations on the key deteriorates, as the value has to be computed for each
"get" operation by applying all the successive merge operations.

FB Task ID: #3428853

Test Plan:
make all check
db_bench --benchmarks=readrandommergerandom
counter_stress_test

Reviewers: haobo, vamsi, dhruba, sdong

Reviewed By: haobo

CC: zshao

Differential Revision: https://reviews.facebook.net/D14991

util/options.cc

@@ -101,7 +101,8 @@ Options::Options()
       table_factory(
         std::shared_ptr<TableFactory>(new BlockBasedTableFactory())),
       inplace_update_support(false),
-      inplace_update_num_locks(10000) {
+      inplace_update_num_locks(10000),
+      max_successive_merges(0) {
   assert(memtable_factory.get() != nullptr);
 }
 
@@ -292,6 +293,8 @@ Options::Dump(Logger* log) const
         inplace_update_support);
     Log(log, "                Options.inplace_update_num_locks: %zd",
         inplace_update_num_locks);
+    Log(log, "                   Options.max_successive_merges: %zd",
+        max_successive_merges);
 }   // Options::Dump
 
 //