add AbandonChannel functionality

[jangorecki]

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Due to inaccuracy of fee estimator that Blixt 0.6.7 is using, it is possible that auto-fee setting will open a channel using very low fee, like 1sat/vB. This will of course result in opening channel transaction being stuck for days/months/years.

This renders Blixt into almost non-functional state. Possible solution (thanks @djkazic for looking into it) is to import seed into LND desktop, double spent transaction, close all channels, transfer onchain on the side, reinstall Blixt using new seed, transfer back to new Blixt wallet. Very far from ideal.

Functionality to AbandonChannel could be easy remedy for such a situation, and possibly other as well, for example when user specify too low fee manually.

[djkazic]

The fee estimator is fixed now.

[jangorecki]

Any resources that could help to onboard new contribution in this issue would be very useful. Like:

• which source file new functionality should be living in?
• other project(s) that already implemented such method? (Having compatible license)
• any other guides that could be useful for new contributors