Client: add heartbeat mechanism and handle connection errors gracefully

gRPC Keepalive:

• https://github.com/grpc/grpc/blob/master/doc/keepalive.md
• https://cs.mcgill.ca/~mxia3/2019/02/23/Using-gRPC-in-Production/

gRPC health check:

• https://github.com/grpc/grpc/blob/master/doc/health-checking.md