Support RPC interface changes at runtime

Now we are able to append methods to interfaces without breaking RPC compatibility with earlier releases. However there is no way that I am aware of to dynamically add entire new RPC interfaces. Methods/parameters are fixed to the class used to instantiate the server at that time. Coprocessors need this. They will extend functionality on regions in arbitrary ways. How to support that on the client side? A couple of options:

1. New RPC from scratch.

2. Modify HBaseServer such that multiple interface objects can be used for reflection and objects can be added or removed at runtime.

3. Have the coprocessor host instantiate new HBaseServer instances on ephemeral ports and publish the endpoints to clients via Zookeeper. Couple this with a small modification to HBaseServer to support elastic thread pools to minimize the number of threads that might be kept around in the JVM.

4. Add a generic method to HRegionInterface, an ioctl-like construction, which accepts a ImmutableBytesWritable key and an array of Writable as parameters.

My opinion is we should opt for #4 as it is the simplest and most expedient approach. I could also do #3 if consensus prefers. Really we should do #1 but it's not clear who has the time for that at the moment.