AMP invoices need to signal:
- AMPRequired in order to avoid being paid by older clients that don't
support it.
- Can't advertised MPP optional, otherwise older clients will attempt
to pay the invoice with regular MPP payment.
Hence, the features advertised on AMP invoices are mutually exclusive
from those advertised on MPP. Create a new set to classify the two.
This commit introduces a feature.Manager, which derives feature vectors
for various contexts within the daemon. The sets can be described via a
staticly compiled format, which makes any runtime adjustments to the
feature sets when the manager is initialized.
