store: Head of store needs to be the *contiguous* head of the store

Currently in main, the `head` of the store is updated when an Append is done (via #186) because the assumption was that the syncer only does contiguous Appends to the store. This is actually **not** true because the syncer can write to the store via Append inside `incomingNetHead` whereby a header that comes in through headersub (or via p2p.Exchange request) can be verified non-adjacently and then applied to the store if it passes the non-adjacent verification.

We need to fix this by ensuring that the store knows of its own highest header that is contiguous (meaning all headers below the `head` are adjacent).

_Note: once the `Tail()` method is implemented, the `head` of the store should be the highest header that is contiguous all the way down to the `Tail().Height()`_