ChannelPool is an abstraction that maintains a pool of gRPC channels, where each channel is uniquely identified by a pair of HostAndPort and ConnectSSLMode. It exposes an interface to create new gRPC stubs: the interface accepts HostAndPort and ConnectSSLMode, creates a new or finds an existing gRPC channel from its pool, updates the last usage of the channel, and returns a unique pointer to a gRPC stub.