Some resources send INDs with non-zero unique transaction ID, which
can conflict with pending RESPs. To fix this, we use two sockets, one
dedicated for INDs, and one dedicated to REQs, NTFs and RESPs.
This new modem API supercedes the old client and server APIs. It
properly queues resource subscriptions and version queries, as well as
caches reachability status for a given resource.
Also, this is a complete redesign of ISI message handling, adding
another layer of abstraction to multiplexing operations to a given
modem resource.