Introduce OnchainTxHandler, move bumping and tracking logic

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.

Author: Antoine Riard