Reject multiple votes from the same IP

[umputun]

The idea is to add an optional restriction treating vote request from the same IP as a vote from the same user. Technically, this is not per IP as we don't store it, but the hash of IP. Either way, this info can be extracted, saved, and analyzed.

This change may not be friendly for users behind some proxy (sharing ip) and will be off by default

[umputun]

One more thing - if we extend voting info anyway, we can also include timestamp which may help to fine-tune same-ip rejection by limiting this check to some predefined time window.

The biggest problem with this task is how to keep it back-compatible with the current Votes map[string]bool stored in bolt for existing comments.