Use a ConcurrentHashMap for deferredDfOpsQueue and deferredDsOpsQueue

[RKrahl]

The deferred operations in the FiniteStateMachine are stored in a HashMap deferredDfOpsQueue or deferredDsOpsQueue respectively depending of storageUnit. In order to maintain thread safety, access to these objects are synchronized, which essentially creates a global lock each time the queue is processed in the FiniteStateMachine. This has been observed to degrade performance in situations where the queue becomes large.

Using a ConcurrentHashMap instead would avoid the need for a synchronized access and might mitigate the degradation.

This proposal has been made by @stuartpullinger in the march 2020 F2F meeting.