docs: refresh electrum protocol links and prune stale TODOs

  The ElectrumX docs (electrumx.readthedocs.io) are frozen ~1.4.2; the
  maintained spec now lives at electrum-protocol.readthedocs.io. Update the
  server.features references in README and the --electrum-hosts help text.

  Also prune two stale TODO.md items:
  - "height -1 for txns with unconfirmed inputs" -- already implemented by
    get_electrum_height.
  - "use generators instead of vectors" -- obsolete; iterators are used
    throughout.

Author: EddieHouston

README.md

@@ -75,7 +75,7 @@ Additional options with the `liquid` feature:
 - `--parent-network <network>` - the parent network this chain is pegged to.
 
 Additional options with the `electrum-discovery` feature:
-- `--electrum-hosts <json>` - a json map of the public hosts where the electrum server is reachable, in the [`server.features` format](https://electrumx.readthedocs.io/en/latest/protocol-methods.html#server.features).
+- `--electrum-hosts <json>` - a json map of the public hosts where the electrum server is reachable, in the [`server.features` format](https://electrum-protocol.readthedocs.io/en/latest/protocol-methods.html#server-features).
 - `--electrum-announce` - announce the electrum server on the electrum p2p server discovery network.
 
 See `$ cargo run --release --bin electrs -- --help` for the full list of options.

TODO.md

@@ -1,12 +1,10 @@
 # Electrum
 
 * Snapshot DB after successful indexing - and run queries on the latest snapshot
-* Update height to -1 for txns with any [unconfirmed input](https://electrumx.readthedocs.io/en/latest/protocol-basics.html#status)
 
 # Rust
 
 * Use [bytes](https://carllerche.github.io/bytes/bytes/index.html) instead of `Vec<u8>` when possible
-* Use generators instead of vectors
 * Use proper HTTP parser for JSONRPC replies over persistent connection
 
 # Performance

src/config.rs

@@ -308,7 +308,7 @@ impl Config {
         let args = args.arg(
                 Arg::with_name("electrum_public_hosts")
                     .long("electrum-public-hosts")
-                    .help("A dictionary of hosts where the Electrum server can be reached at. Required to enable server discovery. See https://electrumx.readthedocs.io/en/latest/protocol-methods.html#server-features")
+                    .help("A dictionary of hosts where the Electrum server can be reached at. Required to enable server discovery. See https://electrum-protocol.readthedocs.io/en/latest/protocol-methods.html#server-features")
                     .takes_value(true)
             ).arg(
                 Arg::with_name("electrum_announce")