Scope: Set new scope prefix on delete

Removes at least one unnecessary cache miss after deleting a scope.
Scope: Remove unimportant TODOs

They aren't bad ideas but they aren't important enough to keep.
Scope: Increase scope prefix length

16^4 = 2^16 = 65536 possibilities isn't very many. This increases it to
16^8 = 2^32 = 4+ billion which is a much more reasonable key space. I'd
like to use something other than md5 to increase the 16 to 62 by using
alphanumeric but it isn't super important at this point.
Benchmarks: Check Memcache availability

This allows benchmarks to be run even if the Memcache extension isn't
available.