Fix bug in MissingValues causing unhandled exception during indexing, since StartPage is
   not defined when there is no request.
Fix various race conditions with and without load balancing by:
 - Changing strategy for provider reloading. Keep provider always loaded and use
   a wrapper instance (TranslationProvider) that can be invalidated and reloaded,
   with simple but effective locking during reloading.
 - Use CacheManager to invalidate translation provider to make it work when load balancing.
 - Simplified and correct use of locks. Removed redundant volatile.
Fix web connection leaks by use of 'using' for bing responses.