Abstract class template for defining a pluggable blocking queue implementation to be used
by the 'pluggable' call queue type in the RpcExecutor.
The intention is that the constructor shape helps re-inforce the expected parameters needed
to match up to how the RpcExecutor will instantiate instances of the queue.
If the implementation class implements the
ConfigurationObserver interface, it will also be wired
into configuration changes.
Instantiation requires a constructor with final int maxQueueLength,
final PriorityFunction priority,
final Configuration conf)
as the arguments.