AbstractFullNodeTest: Fix potential threading issues

- Stop all WOT subsystems which run their own thread to ensure that unit
tests don't have to do thread synchronization: If there is only one
thread (= the unit test) accessing WOT, we don't have to synchronize.
- The existing tests already don't synchronize, so its important to do
this even though I have not run into any threading issues with them yet.
- Also add JavaDoc at WebOfTrust.terminate() which tells people to make
sure that they also add thread termination to unit tests if they
introduce new threads. If you introduce new threads, you'll likely end
up terminating them at the main WebOfTrust.terminate() function, so its
a good place for reminding people.