Instead of passing a Vec of Vecs drop them into one as we go in
ChannelMonitor, hopefully avoiding a bit of memory fragmentation
and improving readability.
Encapsulates tracking and bumping of in-flight transactions in
its own component. This component may be latter abstracted
to reuse tracking and RBF for new features (e.g dual-funding,
splicing)
Build all transactions generation in one place. Also as fees
and signatures are closely tied, what keys do you have determine
what bumping mode you can use.