Fix leaks in writeForever and writeTagsForever
Fix leaks in writeForever and writeTagsForever

writeCachedDataPoints will return a Deferred before it's finished
executing (most noticeable if it needs to wait for a token from
CREATE_BUCKET or UPDATE_BUCKET), so ignoring the return value
before calling it again can result in lots of overlapping calls.

criteo-forks/carbon,piotr1212/carbon,piotr1212/carbon,criteo-forks/carbon,graphite-project/carbon,graphite-project/carbon,obfuscurity/carbon,deniszh/carbon,obfuscurity/carbon,deniszh/carbon