delete idle counters to reduce load on carbon

introduced a new config to the StatsD compliant processor called
"delete-idle-counters". With this config, all counter keys are deleted
on each flush so that we don't send any stat for idle counters. This
reduces the amount of data carbon has to process.
better error handling if counter keys are flushed while a counter is being composed
Merge pull request #3 from bicycleday/master

delete idle counters to reduce load on carbon