only synchronize the write to STLYE_CACHE as the update will be atomic.
Using ConcurrentHashMaps rather than synchronized Maps.
using custom key for looking up from the StyleFormatter cache.

Previously a String was created from the ints, and a locale.toString was
invoked (which is a pretty heavy operation).
Now we use a custom key which is based on the same ints and locale, but
does not need to calculate a string out of it.
Merge branch 'avoid_lock_contention_formatter_cache' of https://github.com/CodingFabian/joda-time into CodingFabian-avoid_lock_contention_formatter_cache
Adjust pull request to use AtomicReferenceArray

Fixes #129, #127