[bbq2]: mpsc notifier isn't quite right

[jamesmunns]

Right now the MPSC impl queues producers if getting the grant fails, but the mpsc notifier queue is only for "not full", but a grant can fail for two reasons:

• the queue is full (this is right)
• a grant is already active

Right now we have no way of notifying that a producer grant is complete other than when a grant has been consumed ("not full"). What we want:

• Task A takes a write grant async, gets it
• Task B tries to take a write async, grant is in progress, task B is queued
• Task A commits the grant
• At this point, Task B should be notified
• If a new task, Task C, tries to take a write grant, it should instead be queued instead of getting the grant before B

This also gets tricky if the queue becomes not-full, BUT there is not enough free space for the requested grant. e.g. Task B wants 128B, but the consumer only released a 64B grant.

[jamesmunns]

note that MPSC notifiers were removed before merging into the bbqueue repo. We may want to bring them back, so keeping this open as a reminder of what needs to be done to do so.