[MASTER ISSUE] Create benchmark/stability tests

It would be useful to have objective tests/benchmarks. These tests/benchmarks can be used to validate future changes to ZooKeeper, compare against other similar products (etcd/consul, etc.) or to help promote ZooKeeper. Possible candidates include:

• leader election tests/benchmarks
• service discovery tests/benchmarks
• distributed locks tests/benchmarks
• ...

Note: each test/benchmark should be a sub-task under this master task