Preload uid caches at startup.

preload uid caches at startup to reduce the latency of fetching many tags for
many spans at query time.
New behavior is enabled by changing opentsdb.conf:
   # preload uid cache at startup
      tsd.core.preload_uid_cache = true

NOTE: Submitted on behalf of third-party contributor:
  * Guenther Schmuelling <schmuell@pepperdata.com>
What's the point of loading everything asynchronously if you're going to block here?  You may as well just remove the `IdLoader` class and implement the cache loading as a single scanning loop in `preload_uid_cache` no?