Rewrite InvoicePayer retry to correctly handle MPP partial failures

Rewrite InvoicePayer retry to correctly handle MPP partial failures

This rewrites a good chunk of the retry logic in `InvoicePayer` to
address two issues:
 * it was not considering the return value of `send_payment` (and
   `retry_payment`) may indicate a failure on some paths but not
   others,
 * it was not considering that more failures may still come later
   when removing elements from the retry count map. This could
   result in us seeing an MPP-partial-failure, failing to retry,
   removing the retries count entry, and then retrying other parts,
   potentially forever.