Pre-allocation of producers

As far as I understood, the constructor:

	ConcurrentQueue(size_t minCapacity, size_t maxExplicitProducers, size_t maxImplicitProducers)

uses the maxExplicitProducers and maxImplicitProducers to set the initial number of blocks. However, it does not preallocate producers (via add_producer). This means that try_enqueue can still malloc a producer when it is called.

Could the constructor mentioned above also include the following snippet in its body to minimise the chance of malloc when using try_enqueue?

for(size_t i = 0; i != maxImplicitProducers; ++i) {
  auto* p = add_producer(create<ImplicitProducer>(this));
  p->inactive.store(true, std::memory_order_relaxed);
}

Start at the ConcurrentQueue(size_t minCapacity, size_t maxExplicitProducers, size_t maxImplicitProducers) constructor and trace add_producer, create, and try_enqueue. Check how producer allocation currently occurs and whether inactive preallocated producers are safe for this queue lifecycle. Done means the constructor preallocates the requested implicit producers and try_enqueue no longer needs to allocate one in the described case.