Add default value to PrioritizedBSyncEventSelectionStrategy

[achiyae]

The default priority value of a bsync is int.min however it should be 0 and also allow the user to set it.

The reason is that in verification, for example, I wish to simulate external event as event with the lowest priority but I cannot do that since all other events already have the lowest value.

Moreover, in many cases you think on priorities as a minimization problem and not maximization, 0 lets you do both...

This issue is probably valid for PrioritizedBThreadEventSelectionStrategy