Add simple rpc throttling

Add a simple version of rpc throttling.
(by simple I mean something that requires less changes as possible to the core)

The idea is to add a hbase:quota table to store the user/table quota information.
Add a couple of API on the client like throttleUser() and throttleTable()
and on the server side before executing the request we check the quota, if not an exception is thrown.
The quota will be per-machine. There will be a flag "QuotaScope" that will be used in the future to specify the quota at "cluster level" instead of per machine. (A limit of 100req/min means that each machine can execute 100req/min with a scope per-machine).

This will be the first cut, simple solution that requires verify few changes to the core.
Later on we can make the client aware of the ThrottlingException and deal with it in a smarter way.
Also we need to change a bit the RPC code to be able to yield the operation if the quota will be
available not to far in the future, and avoid going back to the client for "few seconds".

REVIEW BOARD: https://reviews.apache.org/r/23981