Add fetch-block query (resolves #375)

[thecodefactory]

No description provided.

[coveralls]

Coverage remained the same at ?% when pulling 7c18b5c on thecodefactory:fetch-block into 3303abc on libbitcoin:master.

[thecodefactory]

@evoskuil Is this a sane way to pass the witness bool?

[thecodefactory]

This is probably the better way to go, rather than checking the config setting directly:

    const auto witness = script::is_enabled(
        node.blockchain_settings().enabled_forks(), rule_fork::bip141_rule);

    node.chain().fetch_block(block_hash, witness,
        std::bind(&blockchain::block_fetched,
            _1, _2, request, handler));

[thecodefactory]

Noting here that since it's a new API for v4, we could just hardcode true for this parameter, similar to fetch_transaction2. Alternatively, I could update fetch_transaction2 to use this style so that if the node isn't configured for it, it won't use it (similar to fetch_transaction, but configurable).

I'll probably opt for the latter and we can re-visit and re-organize as needed.