feat(Batcher): Add Limiter option

[bartelink]

Adds a facility to the Batcher that (optionally) extends the Linger period to include a phase during which a SemaphoreSlim is acquired:

• the semaphore is Released when the processing has finished
• further items can join the lingering batch during the acquisition period

[nordfjord]

LGTM, it still surprises me that his stuff isn't available in the stdlib 🥴

[bartelink]

It definitely feels like a cleaner impl should be possible, and I'm also surprised I have not ran into anything similar.
Being able to leave it open while there's batches in flight is definitely a big upgrade on trying 5/10/50/100 and convincing oneself that such a linger time makes sense overall in the context of some anticipated load