store: Streamline Store initialization

[Wondertan]

Currently Store has pretty convoluted initialization process. We detect that store is initialized or not basing presence of Head and allow store to be used when store wasn't Inited.

Options

1. Pass init func in Store ctor:

NewStore(..., initFn func(context.Context) (H, error))

Then during Start, the store is gonna detect if its initialized or not and execute the function accordingly, storing the initial header.

2. Remove notion of initialization entirely:

Initialization simply adds the first header to the Store, so do we really need a separate method/functionality for it, when it can be a simple Append/Put operation on the Store? Furthermore, the initial header is conceptually the same as Tail and considering future pruning this header is gonna move, so the Store doesnt really need to treat this initial header any special.

However, until we have pruning, this approach would require moving the initialization to the Syncer, so it becomes responsible for detecting lack of and fetching the first header. Although, there is nothing fundamentally wrong with this.

3. Apply strict initialization rules

• Start throws error without prior Init
• Store methods throw error without prior Start