Implement burst(max_n) for UPCXX_MPSC_QUEUE_BIGLOCK

[Dan Bonachea]

As described in pull request #312, the UPCXX_MPSC_QUEUE_BIGLOCK implementation of LPC queues (which is off by default and should only be used for debugging runtime correctness problems) lacks a proper implementation of intru_queue<T, intru_queue_safety::mpsc>::burst(max_n, fn).

The best fix (and IMO the only efficient one) is probably to add an operation to intru_queue<T, intru_queue_safety::none> aka unsafe_queue. I see at least two possibilities:

1. A factory operation to "split" an unsafe_queue by dequeing up to max_n entries from a source to construct the temporary queue used in burst, or
2. A "merge" operation that can take the items remaining in the temporary queue after burst and push them onto the front/HEAD of the source queue (maintaining FIFO ordering for fairness reasons).

Approach 1 extends the biglock critical section while traversing up to O(max_n) entries (globally reducing concurrency of the LPC queues), Approach 2 requires re-acquiring the mutex when the burst would exceed max_n, but otherwise has constant complexity.

This is low priority, because nobody should be using this unsupported option for production runs.