Add a `Future` which can receive manager persistence events

Add a `Future` which can receive manager persistence events

This allows users who don't wish to block a full thread to receive
persistence events.

The `Future` added here is really just a trivial list of callbacks,
but from that we can build a (somewhat ineffecient)
std::future::Future implementation and can (at least once a mapping
for Box<dyn Trait> is added) include the future in no-std bindings
as well.

Fixes #1595