Avoid lock contention formatter cache

This PR contains 3 commits improving contention on the caches.

Details on the Change from LinkedHashMap with eviction to a capped ConcurrentHashMap are discussed in https://github.com/JodaOrg/joda-time/issues/127

By switching from a String key to custom key expensive toString conversions of Integers and Locales can be skipped as the hash code & equals of the key is sufficient.

The benchmark showing the performance difference can be found in the following gist:
https://gist.github.com/CodingFabian/9088631