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 can be enabled by changing opentsdb.conf:
    # preload uid cache at startup
    tsd.core.preload_uid_cache = true

Signed-off-by: Chris Larsen <clarsen@euphoriaaudio.com>
Cleanup #326 a bit. Build a map of the UniqueId caches in TSDB and pass that
to the pre-loader instead of building a list then a map.
Pass the TSDB object to the pre-loader and use the config to load the max
results.
Fix naming of UniqueId.preloadUidCache().
Remove the named scope from the pre-load loop and use the row count instead.

Signed-off-by: Chris Larsen <clarsen@euphoriaaudio.com>