Set separate TTL for DNS rewrites

### Problem Description

At the moment DNS rewrites have the same TTL as blocked query. Which is 10 seconds by default. The problem with this approach is that 10 seconds is too low, generates lot more queries, clutters up query log and isn't efficient overall.

Workaround is to set `blocked_response_ttl` to higher value in the config file but that would mean you will have to wait longer whenever a domain is whitelisted in the web gui for browser/client OS DNS cache to time out.

### Proposed Solution

One solution would be is to add another config parameter like `dns_rewrite_ttl` and set the default to say 300 seconds. Rewrites aren't going to change frequently and also something that is manually added by user. This might not need to be exposed in the web gui like how blocked response ttl isn't.

### Alternatives Considered

1. set `blocked_response_ttl` in config file to a higher value.
2. Use separate DNS server for rewrites like dnsmasq or unbound.