[WIP] Make Rene Pay Again

[Lagrang3]

In this PR I would like to make some code refactoring and bug fixes.

Changes:

• separate struct chan_extra and struct flow into two different modules,
• remove unnecessary (and duplicate) functions remove_completed_flow, remove_completed_flowset, commit_flow, and commit_flowset,
• the concept of a payment route is now implemented in a new data structure called struct route, instead of struct pay_flow,
• review the concept of flow: a flow consists of a path and a single amount to be delivered through that path,
• adding a new plugin workflow based on the concept of payment modifiers used in the default pay plugin,

[Sjors]

Suggested release name: Make Rene Pay Again

[Lagrang3]

Suggested release name: Make Rene Pay Again

Good name for the next release.

[Lagrang3]

I've made a rebase on top of PR #7159.