Remove type parameter A from RaftCore

[akiradeveloper]

As of 0.7.1, RaftCore has A: RaftApp as the type parameter but we don't use this. The reason I added this is we can define an associated type A::Snapshot so the App implementer can give the type statically. However, with the abstraction snapshot tag this plan won't be done.

pub struct RaftCore<A: RaftApp>

Another drawback of this design aside being needless will become clear when we implement in-library multi-raft (#131). In there we could have multiple RaftCores that may have apps of different types. With the type parameter, we can't implement this.

[akiradeveloper]

too old.