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().
