Add back Electrum support

[johncantrell97]

In the initial version of Sensei it supported an electrum backend. This was largely implemented in https://github.com/johncantrell97/bdk-ldk .

Haven't thought through how to best integrate it. The individual nodes should not care how they are kept in sync. It probably means redesigning the chain manager / listener interface a bit. It probably means implementing some kind of global 'filter'. Need to brainstorm this one a bit more before tackling it.

The reason to do this is to more easily support running sensei on mobile