Add support for the `blockchain.scripthash.get_mempool` endpoint

[evanlinjin]

This is part of the Protocol Methods.

Currently, if a client attempts to call this, it is met with EOF (which is rather confusing since electrs advertises a protocol version of 1.4).

[EddieHouston]

PR #226 implements blockchain.scripthash.get_mempool.

It returns the unconfirmed history of a script hash as a list of {tx_hash, height, fee}, where height is 0 when all inputs are confirmed and -1 when the tx has unconfirmed parents, per the protocol spec. It performs a mempool-only lookup and enforces the same txs_limit / "too popular" guard as get_history.

On ordering: at protocol 1.4 (which electrs advertises) the result order is undefined, so this PR returns results in mempool order. Protocol 1.6 tightens this to require canonical ordering ((-height, tx_hash), matching the script-hash status order). That's tracked as part of the broader 1.6 work in #172 and can be added here or there.

Thanks @SomberNight for the pointer to the up-to-date docs at spesmilo/electrum-protocol.

[SomberNight]

note: up-to-date protocol docs are available at https://github.com/spesmilo/electrum-protocol/
(rendered at https://electrum-protocol.readthedocs.io/en/latest/protocol-methods.html#blockchain-scripthash-get-mempool)