Significant performance improvements

Fixes an issue that was taking place for the default domain. We had an issue where we weren't allowed to add the default domain to the domain table and would cache miss on each and every redirect links.find call against the default domain.

With the standard Kutt processes, a single redirect request takes between 60 and 80 ms on a local machine, and 120 -150 ms on a small Ubuntu server.

After implementing a cache check on the default domain, a local redirect request dropped to 5-10ms.

I also found that be default there weren't indexes to the links.address column, even though that is where the query primarily takes place. Adding an index to this a local redirect request (without the cache optimizations in place) took between 7-15 ms for a read.

• Adds indexes on database for significantly faster lookups.
• Adds a check for default domain against the cache. As default domain can't be added to the custom domains, we need to have a default domain check for cache