frontend-grpc: limit the number of request processing threads

Summary

if a large number of requests are submitted to CTA, then frontend will spawn too many threads that eventually 'kill' the service.

This change introduces a thread number limit, which defaults to 8xCPUs, if not explicitly defined during the start.

Requires manual tests in pre-production

NO

Closes #318 (closed)

It would be nice to get this change backport into 5.7 branch