Clarify call handling thru invite
This is about moving call.hpp/call.cpp to invite.hpp/invite.cpp, as we really only track state for the initial invite. When the first answer arrives, all other invites are cancelled, and we receive the final bye from the ua thru record-route. This really only covers the existing headers & class names for consistency, not the actual operations; that will come later.
Edited by username-removed-233259