Replace maze of BOLT11 payment utilities with parameter generators

`lightning-invoice` was historically responsible for actually
paying invoices, handling retries and everything. However, that
turned out to be buggy and hard to maintain, so the payment logic
was eventually moved into `ChannelManager`. However, the old
utilites remain.

Because our payment logic has a number of tunable parameters and
there are different ways to pay a BOLT11 invoice, we ended up with
six different methods to pay or probe a BOLT11 invoice, with more
requested as various options still were not exposed.

Instead, here, we replace all six methods with two simple ones
which return the arguments which need to be passed to
`ChannelManager`. Those arguments can be further tweaked before
passing them on, allowing more flexibility.

Author: TheBlueMatt