This commit adds handling code for the key send custom record. If this
record is present and its hash matches the payment hash, invoice
registry will insert a new invoice into the database "just in time". The
subsequent settle flow is unchanged. The newly inserted invoice is
picked up and settled. Notifications will be broadcast as usual.
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.
This commit adds Clock and DefaultClock and moves the private
invoices.testClock under the clock package while adding basic
unit tests for it.
Clock is an interface currently encapsulating Now() and TickAfter().
It can be added as an external dependency to any class. This way
tests can stub out time.Now() or time.After().
The DefaultClock class simply returns the real time.Now() and
time.After().
