Log IP address when scheduling payout
Add payout_interval info to main page
Fix stats time skew

Subtract skew instead of adding it, to cancel the error instead of
doubling it.
CoinBank: fix race in schedule_payment()

The schedule_payment() method checks several conditions before
scheduling a payment. The original order was essentially:

- Check address validity (async)
- Check for duplicates (sync)
- Get amount (async)
- Schedule payment (sync)

The interleaving of asyncrhronous operations with synchronous operations
renders the check for duplicates unreliable. The order should be:

- Check address validity (async)
- Check amount (async)
- Check for duplicates (sync)
- Schedule payment (sync)

This ensures that nothing happens between checking for duplicates and
updating the payment data structures.