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
