Expose db stress tests (#5937)

Summary:
expose db stress test by providing db_stress_tool.h in public header.
This PR does the following:
- adds a new header, db_stress_tool.h, in include/rocksdb/
- renames db_stress.cc to db_stress_tool.cc
- adds a db_stress.cc which simply invokes a test function.
- update Makefile accordingly.

Test Plan (dev server):
```
make db_stress
./db_stress
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5937

Differential Revision: D17997647

Pulled By: riversand963

fbshipit-source-id: 1a8d9994f89ce198935566756947c518f0052410

Makefile

@@ -1165,7 +1165,7 @@ memtablerep_bench: memtable/memtablerep_bench.o $(LIBOBJECTS) $(TESTUTIL)
 filter_bench: util/filter_bench.o $(LIBOBJECTS) $(TESTUTIL)
 	$(AM_LINK)
 
-db_stress: tools/db_stress.o $(LIBOBJECTS) $(TESTUTIL)
+db_stress: tools/db_stress_tool.o tools/db_stress.o $(LIBOBJECTS) $(TESTUTIL)
 	$(AM_LINK)
 
 write_stress: tools/write_stress.o $(LIBOBJECTS) $(TESTUTIL)

include/rocksdb/db_stress_tool.h

@@ -0,0 +1,9 @@
+// Copyright (c) 2013-present, Facebook, Inc.  All rights reserved.
+//  This source code is licensed under both the GPLv2 (found in the
+//  COPYING file in the root directory) and Apache 2.0 License
+//  (found in the LICENSE.Apache file in the root directory).
+#pragma once
+
+namespace rocksdb {
+int db_stress_tool(int argc, char** argv);
+}  // namespace rocksdb