Update client to support Electrum protocol v1.6 methods

[evanlinjin]

To Do

Comment below if this list is incomplete.

• server.version should tolerate and ignore extraneous arguments.
• Add optional mode argument for blockchain.estimatefee.
• blockchain.block.headers now returns headers as a list. Custom parsing logic can be implemented to support both the new and old responses.
• Add blockchain.transaction.broadcast_package.
• Add mempool.get_info().

Other notes

I think we should make this layer unopinionated for simplicity. The caller will be responsible for making the right calls. I.e. send server.version as the first message.

Context

bitcoindevkit/rust-electrum-client#189