Overhaul SIP implementation

[doe300]

The current SIP implementation has a few weak points, namely:

• session state: absolutely inaccurate and obscure, does not consider REGISTER, is set at various places, etc.
• request-handling: currently all requests are standalone, with no connection to a session/etc.
• local user agent: initially set to local user and device, switching to another user (e.g. for registration) not clearly structured/tested
• only one action possible: if program registers to a server, it can't invite anyone
• multi-destination: feature to add multiple destinations partly added, won't work at all, since implementation aims at a single destination

Rewrite to fix this:

• Add SIPCallflow which handles all requests/responses, until the call-flow is ended
• Add subclasses for call-flows such as registration (including refreshment, deregistration), invitation (session-establishment, ..., up to closing), handling OPTIONS-requests, ...
• Make sure call-flows and can coexist (e.g. refreshing registration and handling session, or handling session and answering to OPTIONS-request)
• remove code supporting multi-destination add correct support for multiple destinations with distinction, where a single destination/multiple destinations is/are required
• Overhaul check for valid headers/SIP-URIs, IP-addresses