Allow custom service names with srvServiceName URI option

DRIVERS Ticket Description We are using DCOS (marathon/mesos) to manage our services. DCOS generates SRV records for our mongos instances under a record that looks like _mongos-mongodb._tcp.marathon.mesos However, there's currently no way for me to use this because when I provide a connection url like mongodb+srv://_mongos-mongodb._tcp.marathon.mesos the drivers prepend " mongodb._tcp" to the provided url. It's not clear why it's required that the host _must start with "_mongodb._tcp". Why not let the user specify the actual DNS entry to query? I believe the same issue will exist for multiple orchestration frameworks such as Consul/Nomad and Kubernetes. Drivers should sync their URI options and seedlist discovery spec tests to this commit . Drivers should accept a new URI option srvServiceName that overrides the default "mongodb" service name for SRV lookup in both discovery and polling.