Follow up on maintaining connections for grpc

[guanw]

Current grpc resolver used by agent to resolve collector address doesn't have the ability to maintain connections like what peer_list_mgr.go did using maintainConnections. Need to investigate if it's necessary to do so with grpc and if it indeed is how to do it.

Originally posted by @yurishkuro in #1498

[yurishkuro]

I am not sure if we need something explicit for that or not. A way to try this out (could be in the unit tests) is to do this:

• use fixed discoverer w/ 5 servers and minPeers=3
• start sending requests, verify all 3 servers are receiving them
• shut down one of the servers, verify that all requests are going to the other two servers
• bring up the 3rd server again (must be on the same port), verify all 3 servers are receiving requests

[guanw]

Tested it. Roundrobin will include servers in this case. Cheers.