Handle empty WP_CACHE_SALT parameter

[soulseekah]

As per 7b85660 from #13 by @msigley we seem to prefix the keys with a single ":" if the WP_CACHE_SALT parameter is empty.

The proposal is to remove the prefix altogether then.

Is this okay? Is this not okay?
If we change it now do we break existing deployments? Probably not. They'll just expire. Any conflicts? Who knows?

[soulseekah]

This is a great idea 853454c

define( 'WP_CACHE_KEY_SALT', ABSPATH ); by @kagg-design

I wouldn't think that ABSPATH would be a good key prefix in Redis, though.

Maybe hash it to something less obtuse?

Maybe we can always use ABSPATH in the prefix, outside of the WP_CACHE_KEY_SALT context?

Needs more thoughts.

[msigley]

I hashed the abspath with crc32 in my implementation. Big key lengths are bad for redis performance.
Removing the : on empty cache salts allows the redisinsight memory profiler to function properly.