Implement the ability to block on multiple futures at once

Implement the ability to block on multiple futures at once

In the next commits we'll be adding a second notify pipeline - from
the `ChainMonitor` back to the background processor. This will
cause the `background-processor` to need to await multiple wakers
at once, which we cannot do in the current scheme without taking on
a full async runtime.

Building a multi-future waiter isn't so bad, and notably will allow
us to remove the existing sleep pipeline entirely, reducing the
complexity of our wakers implementation by only having one notify
path to consider.