Short-Circuit Coprocessor HTable access when on the same server

Coprocessors currently create a full HTable when they want to write. However, we know that coprocessors must run from within an HBase server (either master or RS). For the master, its rare that we are going to be doing performance sensitive operations, but RS calls could be very time-intensive.

Therefore, we should be able to tell when a call from a CP attempts to talk to the RS on which it lives and just short-circuit to calling that RS, rather than going the long way around (which does the full marshalling/unmarshalling of data, as well as going over the loopback interface).