This commit intends to fix slow first startup time when there are many
invoices that need to be canceled. The slowdown is caused by a combination
of adding invoices to the expiry watcher one-by-one and slow
cancellation. Due to slow cancellation and the unbuffered channel which
we use to pass invoices to the expiry watcher blocks the registry.
With this fix we'll instead batch add invoices to the expiry watcher and
thereby won't block the registry startup.
This commit adds InvoiceExpryWatcher which is a separate class that
receives new invoices (and existing ones upon restart) from InvoiceRegistry
and actively watches their expiry. When an invoice is expired
InvoiceExpiryWatcher will call into InvoiceRegistry to cancel the
invoice and by that notify all subscribers about the state change.
