Set/Sorted Test Key Prefix Consistency

The Set and Sorted Set tests were not using PrefixedKey() consistently.
As a result, some sets were not being torn down properly between tests.

This prevented the Set and Sorted Set tests from being run in
succession, since some sets used in the Set tests still existed when
Sorted Set tests were run.  The result is an error from Redis, since
Sorted Set operations(e.g. ZADD) cannot be applied to a regulart Set.

The change is to used PrefixedKey() wherever necessary.  SetId has been
changed to a property that uses PrefixedKey(), so most tests were
automatically fixed.