A mapped queue, similar to Thread::Queue, except that as elements
are queued, they are assigned unique identifiers, which are used
to identify responses returned from the dequeuing thread. This
class provides a simple RPC-like mechanism between multiple client
and server threads, so that a single server thread can safely
multiplex requests from multiple client threads. Note that simplex
versions of the enqueue methods are provided which do not assign
unique identifiers, and are used for requests to which no response
is required/expected.

In addition, elements are inspected as they are enqueued/dequeued
to determine if they are Thread::Queue::Queueable (aka TQQ)
objects, and, if so, the onEnqueue() or onDequeue() methods are
called to permit any additional class-specific
marshalling/unmarshalling to be performed. Thread::Queue::Duplex
(aka TQD) is itself a Thread::Queue::Queueable object, thus
permitting TQD objects to be passed between threads.