Tweaking log calls a bit.

-) Introducing gpr_vlog so to spare a few vsprintf later (at least one for now)
-) Renaming statistics/log.* to statistics/census_log.* to avoid collisions.
	Change on 2014/12/12 by nnoble <nnoble@google.com>
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=81995756