Add RpcClient interface and enable changing of RpcClient implementation

Currently HConnectionImplementation works with the included RpcClient which is a direct implementation and not defined by an interface.

It would be great to be able to swap out the default RpcClient with another implementation which can also be controlled by the default HConnectionImplementation.

Suggested changes:

• Create a RpcClient interface which defines all the ways HConnectionImplementation interacts with an RPC client. Like getting a blocking protobuf service interface or closing the client.
• Define which RpcClient implementation to construct by setting a configuration variable which defaults to the current RpcClient.
• Possibly create an abstract RpcClient class to only load all the basic Rpc layer configurations to be used in an implementation.

Why? It enables experimentation with RpcClients which could enable new features or could be more performant than the included client.
I created a new RpcClient implementation based on Netty which can also be called asynchronously. It would be great to also be able to use this RpcClient in all the default ways and tests to see if there are any issues with it.

https://github.com/jurmous/async-hbase-client/
https://github.com/jurmous/async-hbase-client/blob/master/src/main/java/org/apache/hadoop/hbase/ipc/AsyncRpcClient.java