Avoid locking as much as possible related to Chronology caching.

All Chronology classes used synchronising on the chronology cache, which
leads to degraded performance once the cache has a high hit ratio.
This patch narrows the synchronised path onto the updating part by using
ConcurrentHashMap. The remaining synchronisation ensures identity and
prevents lost updates, which might be acceptable but are not strictly
backwards compatible.
Simplified Cache Lookup of GJChronologies.

This commit replaces the cache lookup using only the zone and then
iterating over the ArrayList donating possible candidates with a single
cache access. For this purpose a temporary cache key is instantiated.

In the cache hit case this code will produce a temporary GJCacheKey
object and then return the GJChronology, as compared to previous, where
the code was forced synchronised, an ArrayList iteration was performed
and an Instant was checked for equality.
Merge branch 'avoid_lock_contention_chonology_cache' of https://github.com/CodingFabian/joda-time into CodingFabian-avoid_lock_contention_chonology_cache
Increased performance of chrnology lookup

Fixes #126