Setting `max_path_count` to `channel_manager.list_usable_channels().len()` fails to route full amount

[MaxFangX]

For convenience, here's the scenario in #3685 which is fixed by #3707:

• Alice -> Bob 200k and 300k sat channels
• Bob -> Charlie 600k sat private channel
• Send a MPP payment Alice -> Charlie of size 350k sats

If we amend this scenario so that:

• Alice limits max_path_count to channel_manager.list_usable_channels().len() (i.e. 2)
• Alice tries to send 450k sats instead of 350k sats

Then the regression test added in #3707 fails.

Repro here: https://github.com/lexe-app/rust-lightning/tree/max/repro-max-path-count-limit

[TheBlueMatt]

Right, this is because we require each MPP path to pay at least amount / max_path_count, so if one channel can only pay a bit less we'll refuse to use it. max_path_count isn't really intended to reduce MPP, only turn it on/off, though maybe we could allow for additional MPP path finding steps to make this work.