DATAMONGO-830 - Prevent NullPointerException during cache warmup in CustomConversions.

We now use a ConcurrentHashMap to cache the results of custom read target lookups in order to avoid having to traverse the readingPairs for every lookup. The use of ConcurrentHashMap should also prevent potentially NullPointerExceptions from being thrown if custom conversions are initialized in heavily threaded environments.

Original pull request: #117.