Make storage futures only borrow client, not self, for better ergonomics

[jsdw]

Just a small qol improvement that allows one to get a future back from a storage query and await it later without needing to store any intermediate variables due to unnecessarily large lifetimes being captured by said futures.

Without this you'd need to write:

let staking = api.storage().staking();
let a_fut = staking.bonded(&addr, None);
let b_fut = staking.ledger(&addr, None);
let (a, b) = join!(a_fut, b_fut);

But with it you can chain all the way up to the future like so:

let a_fut = api.storage().staking().bonded(&addr, None);
let b_fut = api.storage().staking().ledger(&addr, None);
let (a, b) = join!(a_fut, b_fut);

Closes #558

[niklasad1]

👍

[jsdw]

Note to self; key args have a lifetime of 'a, which is the same as the lifetime of the client struct.

Should key args have a lifetime of 'b, and the returned future rely on both 'a and 'b, to avoid any unnecessary restrictions on how the future is used in conjunction with these references?

It would be good to test and see whether a single bound is overly restrictive (as well as to test that we can create and later use futures so that we confirm it's all good in one place.

[jsdw]

I did a little testing and it seems OK to use the 'a lifetime for key_args and client; from reading around I think Rust will pick the correct smallest lifetime that all params with 'a use anyway, so no need to add other lifetimes.