Contact Introduction Backend
This MR allows you to introduce two of your contacts to each other. They both will receive an introduction with an optional message and then can accept or refuse the introduction which is presented as a notification.
When reviewing, I propose to review the individual commits separately as I took great care to split functional independent parts into separate commits. You might also want to have a look at the [Introduction Client Wiki page](https://code.briarproject.org/akwizgran/briar/wikis/IntroductionClient) to better understand what is going on before looking into the actual code.
Protocol sessions and states are not yet deleted and the UI is still missing (#253). In order to practically test this feature, the UI from !122 is needed.
See merge request !116
This Introduction BSP Client uses its own group to communicate with
existing contacts. It uses four types of messages to facilitate
introductions: the introduction, the response, the ack and the abort.
The protocol logic is encapsulated in two protocol engines, one for the
introducer and one for the introducee. The introduction client keeps the
local state for each engine, hands messages over to the engines and
processes the result and state changes they return.
This requires exposing the `containsContact()` method to the `DatabaseComponent`
and is needed for finding out efficiently whether a contact already exists.
