Make AuthorityId generic (#1296)

* BlockAuthorityId convenience type

* Rename AuthorityId -> Ed25519AuthorityId to make it more precise

* Generalize AuthorityId up to substrate-client

* Fix in client-db

* rename: BlockAuthorityId -> AuthorityIdFor

* typo: should be digest item

* Fix test-runtime authorityId mismatch

One states that AuthorityId is u64 while the other states that it's Ed25519AuthorityId.

* Fix more u64 - Ed25519AuthorityId mismatch

* Fix compile of most of the srml modules

* Continue to pin aura and grandpa with ed25519 and fix compile

* Add MaybeHash trait

* Fix node-runtime compile

* Fix network tests