Added clear subcommand for control lock with following options:
```
3. Clear lock named 'bucket/object' (exact match).
$ minio control lock clear http://localhost:9000/bucket/object
4. Clear all locks with names that start with 'bucket/prefix' (wildcard match).
$ minio control lock --recursive clear http://localhost:9000/bucket/prefix
5. Clear all locks older than 10minutes.
$ minio control lock --older-than=10m clear http://localhost:9000/
6. Clear all locks with names that start with 'bucket/a' and that are older than 1hour.
$ minio control lock --recursive --older-than=1h clear http://localhost:9000/bucket/a
```
These messages based on our prep stage during XL
and prints more informative message regarding
drive information.
This change also does a much needed refactoring.
- Instrumentation for locks.
- Detailed test coverage.
- Adding RPC control handler to fetch lock instrumentation.
- RPC control handlers suite tests with a test RPC server.
This PR contains various fixes for the distributed release:
- Use DRWMutex in namespace-lock only for a single Lock()/RLock() call in conformance to server-side rw-locking as implemented in minio/dsync
- Implement missing cases in lock-rpc-server to catch Unlock() for active read locks and RUnlock() for an active write lock
- Refactor RPCClient to release local mutex while making actual RPC.Call()
