CoinBank: initialize paid with current time

Instead of setting the time to zero, use core.time() during CoinBank
initialization.
CoinBank: extend get_public_status()

Add start_time and total_pay_amount.
Add total paid to stats page

Report the cumulative payout total on the stats page.
Support persistent payment history.

If faucet.history_file exists, then treat it as a pickled version of
bank.paid:

- CoinBank will attempt to load it on initialization
- make_payments() will update it on each payout
Explain faucet.reuse_addr better in comments

The existing comment made no mention of the most important aspect of
this option: it can be set to an address manually.