Allow resteasy to handle scoping in CandlepinSingletonScope

Resteasy can reuse threads, so it's probably best not to use
ThreadLocal here.

This probably isn't an issue becasue Scope.exit() clears out
ThreadLocal data. I'm not sure if there's any way exit can be
missed, if so we're better off letting RestEasy handle this.
Merge pull request #527 from candlepin/ckozak/clean_cpsingleton_scope

Allow resteasy to handle scoping in CandlepinSingletonScope