Fix startup failure with 0-conf mempool transaction

* updatePaymentWithTransaction() was lacking a callback when paymentChanged
  is false.  The missing callback was non-fatal for other users (jobs and
  walletnotify/blocknotify) so we didn't see the failure.
* The second parameter of listSinceBlock with bitcore does not seem to work as
  documented to limit the output to that number of confirmations.  Remove it.
* Clarify the comments in updatePayment a bit.