Answer general queries with the random delay #202

General queries (?APRS?, ?IGATE?, ?WX?) go to everyone, so every station answering at once would flood the channel. APRS101 asks for a random delay, and for an answer only when the query's area includes the station.

Scope:

• Recognise general queries, including the optional area (?APRS? lat,lon,radius).
• Decide whether to answer, and when: a random delay inside the spec's window, with TimeProvider and an injectable random source for tests.
• ?IGATE? answers include the IGate's counts, ready for the receive-only IGate in this milestone.

Done when:

• tests cover each general query, a query whose area leaves the station out, and the delay window;
• the API is listed in PublicAPI.Unshipped.txt.