Allow specifying maximum number of connections on Server

This will probably be implemented as part of a larger piece of work to control resources on server-side.

http://stackoverflow.com/questions/37338038/how-to-configure-maximum-number-of-simultaneous-connections-in-grpc

Note that slightly different metrics (like maximum number of streams across the server) may be better; I'm not trying to dictate the exact metric, just the need for this sort of control.