Feature: Support Electrum protocol v1.6

**Is your feature request related to a problem? Please describe.** The Electrum protocol v1.6 was just merged. See [the docs](https://electrum-protocol.readthedocs.io/en/latest/protocol-changes.html#version-1-6) for changes in the new version: > **Changes** > - Breaking change for the version negotiation: we now mandate that the server.version() message must be the first message sent. That is, version negotiation must happen before any other messages. > > - Also for server.version(), the server must tolerate and ignore extraneous arguments, to allow for extensions in future protocol versions. > > - The status of a scripthash has its definition tightened in a backwards-compatible way: mempool txs now have a canonical ordering defined for the calculation (previously their order was undefined). > > - blockchain.scripthash.get_mempool() previously did not define an order for mempool transactions. We now mandate a specific ordering. > > - Optional mode argument added to blockchain.estimatefee(). > > - blockchain.block.headers() now returns headers as a list, instead of a single concatenated hex string. > > **New methods** > - blockchain.transaction.broadcast_package() to broadcast a package of transactions (submitpackage). > > - mempool.get_info() to get more detailed and general relayfee info. > > **Removed methods** > - blockchain.relayfee() is removed. The minrelaytxfee field of the new mempool.get_info() RPC is a direct replacement. **Describe the solution you'd like** It would be great if `electrs` could support the new features very soon. For us (LDK), support for the new `blockchain.transaction.broadcast_package` is particularly important, as we want to lean on TRUC/v3 transaction relay ASAP.