Fixes ExclusivityManager synchronization bug (#545)
Fixes ExclusivityManager synchronization bug (#545)

Fixes #543 

The ExclusivityManager was using `DispatchQueue.initiated`, which
returns a global **concurrent** queue, however its internals are
dependent upon **serial** execution of add/remove.
Modified to use a serial queue.
ProcedureKit/ProcedureKit,blg-andreasbraun/ProcedureKit,blg-andreasbraun/Operations,blg-andreasbraun/ProcedureKit,danthorpe/Operations,danthorpe/Operations,blg-andreasbraun/Operations,danthorpe/Operations,blg-andreasbraun/ProcedureKit,ProcedureKit/ProcedureKit,blg-andreasbraun/Operations,ProcedureKit/ProcedureKit