Convert sample denominator from hard-coded constant to configurable
value, remove unused global config option 'decoder_poolsize'.
Update docs to reflect global config changes.
Validate sample_denominator > 0, and make sure we actually apply it
to the globals.
Merge pull request #766 from mozilla-services/configurable-sample-denom

Convert sample denominator from hard-coded constant to configurable
Merge branch 'dev' into issue_677

Conflicts:
	CHANGES.txt