Allow coprocessor callback RPC calls to be batched at region server level

Cuurently the Callback.update() method is called for each Call.call() return value obtained from each region. Each Call.call() invocation is a separate RPC, so there is currently one RPC per region. So there's no place at the moment for the region server to be involved in any aggregation across regions.

There is some preliminary support in HConnectionManager.HConnectionImplementation.processBatch() that would allow doing 1 RPC per region server, same as we do for multi-get and multi-put.

We should provide ability to batch callback RPC calls.