cluster: apply node hint before filtering by latency.

Otherwise we could get in a state where we select the wrong node even
though we know which node we want to communicate with.
cluster: implement mongoc_cluster_try_send().

This will return 0 on failure, or the index of the node communicated with
starting from 1. This value can be supplied to a supplimental
mongoc_cluster_send() or mongoc_cluster_recv() to communicate via the
same node.