Allow sending CreateTimeSeriesReq in multiple goroutines (#213) * Allow sending CreateTimeSeriesReq in multiple goroutines * Small fix * Fix error handling, add tests * Use shared reqs channel across all workers * Use shared response channel across all workers * Revert shared reqs channel across all workers * Use WaitGroup instead of quit chan, use shared reqsChan * Make metricsBatcher have at least one worker * Make reqsChan larger, remove an unused check
