Expose checkQuota to Coprocessor Endpoints

Coprocessor endpoints may do non-trivial amounts of work, yet quotas do not throttle them. We can't generically apply quotas to coprocessors because we have no information on what a particular endpoint might do. One thing we could do is expose checkQuota to the RegionCoprocessorEnvironment. This way, coprocessor authors have the tools to ensure that quotas cover their implementations.

While adding this, we can update AggregationImplementation to call checkQuota since those endpoints can be quite expensive.