Remove extra logging statement thats no longer needed.

When you turn on debug logging in Puppet, this statement shows
up hundreds of times for all of the automatic and implicit
hiera() looukps that happen in Puppet. Quiet this down.
Add a local in-memory cache to the rs_tag backend.

This stores frequently looked up tag lists in-memory until the cache
expires on each key. Prevents duplicate lookups of frequently accessed
keys.
Add debug logging statements to cache code.
Bugfix: Typo
Merge pull request #3 from Nextdoor/caching

Caching