This commit extends the existing invoiceRegistry functionality to wrap
the on-disk invoices available via the channeldb with an in-memory
cache on invoices. Currently the in-memory cache is only reserved for
the storage of special “debug” invoices which all nodes are able to
settle immediately.
This commit introduces the invoice registry which is a central
repository of all outstanding invoices related to the daemon.
This registry will be used by the goroutines which manage the htlc’s
for a particular channel, and later by the point-to-point workflow
which negotiates the conditions for payment along with signed recipes.