Fix possible deadlock in neuron update loop in multithreaded code (fixes #718)

[heplesser]

This PR fixes a potential deadlock when a neuron throws an exception during multithreaded simulation (#718).

Since it is difficult to test for deadlocks in unittests, there is no automated test. This manual test is inspired by @maharjun's deadlock module. To test, add the following to the beginning of iaf_psc_alpha::update():

 if ( this->get_thread() == 2 )
	  throw KernelException("Thread 2");

and run the following SLI script from a file

0 << /local_num_threads 4 >> SetStatus
/iaf_psc_alpha 10 Create ;
10 Simulate

NEST should hang if you use current master; with the correction given here, NEST should terminate with an error message.

@maharjun Would you test and review this fix?

[maharjun]

I have tested it and can confirm that this works correctly.

[sanjayankur31]

Looks OK to me. 👍

[heplesser]

Merging as tests pass and two reviewers confirmed that the deadlock no longer occurs.