Beginning now I will begin integrating blockchain database changes
inside libbitcoin_blockchain, afterwards porting obelisk and sx on the
develop branches.

New methods I propose:

    fetch_block_data(...) -> block_header + [tx_positions]
    fetch_transaction(position_type position, ...)
    scan(prefix, handler, from_height, to_height)

    fetch_history will have stop and start indexes.
    (* or maybe 2 methods)

Also there will be a distinction between required and optional databases.
Optional databases:
* history database
* history scan database
* stealth database
You will have the option to activate these after certain block heights
making initial sync much faster.