This is necessary so we can build the absolute locktimes in the next step. For
now just fetch the blockheight on each (sub-)payment, later we can reuse the
root blockheight if it ends up using too much traffic.
A payment is considered finished if it is in a final state (success or
failure) or all its sub-payments are finished. If that's the case we notify
`payment_finished` and bubble up through `payment_child_finished`, eventually
bubbling up to the root, which can then report success of failure back to the
RPC command that initiated the whole process.
This is likely a bit of overkill for this type of functionality, but it is a
nice first use-case of how functionality can be compartmentalized into
modifiers. If makes swapping retry mechanisms in and out really simple.
This should make it easy for JSON-RPC functions and modifiers to get the
associated data for a given modifier name. Useful if a modifier needs to
access its parent's modifier data, or in other functions that need to access
modifier data, e.g., when passing destination pointers into the `param()`
call.
This commit can be reverted/skipped once we have implemented all the logic and
have feature parity with the normal `pay`. It's main purpose is to expose the
unfinished functionality to test it, without completely breaking the existing
`pay` command.
