Parallelize execution of multi operations on RegionServer

In the context of HBASE-10169, we're adding the ability to batch Coprocessor endpoint calls per regionserver, using the same batching that happens in the RegionServer.multi() calls. However, execution of each of the calls will still happen serially on each RegionServer. For Coprocessor endpoint calls, it might help to parallelize these, since each execution could be of indeterminate length.

Since it may help to parallelize the Coprocessor endpoint invocations, it raises the question of whether other operations handled in multi() calls should also be parallelized, or should we just rely on macro-scale parallelization through the RPC handler threads?