In some case, we can get ```java.lang.IllegalStateException: Pool not open``` when Thread ```jmxtrans-export-1``` calls write(...) on a GraphiteWriter that was previously closed.
In order to prevent that - as much as possible - we can shutdownNow() the ScheduledExecutorServices so that tasks that are currently running can detect that the Thread is interrupted, and eventually terminate earlier.

… state of each of the OutputWriters is maintained by OutputWriterSet class, that also provided way to ensure consistent write on all OutputWriters even when some of them are failing. Methods ```startAll()```, ```stopAll()``` and ```writeAll()``` are mutually synchronized, so that we neither stop OutputWriters while writing on it, nor write with OutputWriter while or after they are stopped.

…exported, so that in case of an Excpetion during the export, we try again to export this batch on the next export invocation