Better lifetime bounds for traits methods that return futures

[matko]

Many of the trait methods that return futures return a future with a static lifetime bound. This requires these futures to take ownership of any data in self they require through cloning. While we usually can do that, since many of the data structures we actually use are wrapped in an Arc, it is actually unnecessary. By specifying that the future we return has the same lifetime as the self reference, it should be possible to write implementations that don't need to take ownership.

Trait methods should be rewritten to have these better lifetimes.

[matko]

the async_trait macro does this for free and should just be used everywhere.