Switch to async keystore use

[JoshOrndorff]

So far we use the legacy SyncCryptoStore. I made this decision intentionally so I could copy code from Aura when I was really struggling with the keystore. Now that things are working we should try to go async again.

Not really necessary, but it might be nice to solve #16 first so the code gets simpler.

[nanocryk]

Some of the usages of the CryptoStore are made in the impl of the ConsensusDataProvider trait, which is not async.

The only function that can be made async is first_available_key which is only used in ParachainConsensus::produce_candidate which is async.

Not sure it is worth it until ConsensusDataProvider is async.