Provide lookup-type alternative to subscriptions

[m-ronnblom]

Certain applications are only interested in what services are available at a particular moment. The way to perform such a lookup-type operation with the current API is to issue a paf_subscribe() and cancel it more-or-less immediately.

However, there is no way for the application to determine an appropriate timeout. This is not so such much of a problem if the lookup is not latency-sensitive, since the timeout can be set to a large value (e.g., one second).

For application that are latency sensitive, a function paf_lookup(), providing a snapshot of the currently-matching services for a particular filter, would make a lot of sense. The Pathfinder client library is in a better position to know what is a suitable timeout (although it might also struggle with the question what is an appropriate value).

The protocol-level facility for this operation is already in place in the form of the "services" command.