Asyncify keystore

[rakanalh]

This is part of the work that has to be done in order to implement #4689.

The goal of this issue is to:

1. convert the trait BareCryptoStore to define async functions.
2. Rename the current Store in sc_keystore to LocalStore and update the interface accordingly.

#4689 Would then introduce RemoteStore which implements BareCryptoStore and is able to relay key management to the remote signer.

I am separating both issues so that i can merge smaller, easier to merge PRs and to prevent the scope of the parent issue from getting bigger.

[burdges]

How is the keystore structured now? It already does the signing itself?

[rakanalh]

The keystore currently lives in it's own crate here: https://github.com/paritytech/substrate/blob/master/client/keystore/

And through #4925, we made signing go through the keystore instead of using the pairs directly to enable the introduction of a remote keystore.

[rakanalh]

This was done in #7000