Simplify Tracker to use Boolean as state

[jwstegemann]

Until now, a tracker could distinguish several different transactions that could be passed as a parameter to the track function. This rarely needed functionality can be implemented by using multiple trackers (whose data streams can be combined if needed). This can significantly simplify the tracker's implementation.
Specifying a transaction as a parameter of track is no longer possible. Appropriately, no defaultTransaction can be defined in the factory either. Likewise, the possibility to get a flow, which checks whether a certain transaction is running, by invoking the Tracker is omitted.

Solves #736