Hold the `total_consistency_lock` while in `outbound_payment` fns

Hold the `total_consistency_lock` while in `outbound_payment` fns

We previously avoided holding the `total_consistency_lock` while
doing crypto operations to build onions. However, now that we've
abstracted out the outbound payment logic into a utility module,
ensuring the state is consistent at all times is now abstracted
away from code authors and reviewers, making it likely to break.

Further, because we now call `send_payment_along_path` both with,
and without, the `total_consistency_lock`, and because recursive
read locks may deadlock, it would now be quite difficult to figure
out which paths through `outbound_payment` need the lock and which
don't.

While it may slow writes somewhat, it's not really worth trying to
figure out this mess, instead we just hold the
`total_consistency_lock` before going into `outbound_payment`
functions.