Thread handling after exiting QuickFIX

Hi,

I stumbling over this thing over and over. When I stop my application I always get error messages in output about spawning a new thread during execution of application but failing to remove it when application stops:

```
06-Nov-2015 12:29:26.653 SEVERE [localhost-startStop-2] org.apache.catalina.loader.WebappClassLoaderBase.checkThreadLocalMapForLeaks The web application [ROOT] created a ThreadLocal with key of type [java.lang.ThreadLocal] (value [java.lang.ThreadLocal@682d21a6]) and a value of type [quickfix.field.converter.UtcTimestampConverter] (value [quickfix.field.converter.UtcTimestampConverter@7f03e077]) but failed to remove it when the web application was stopped. Threads are going to be renewed over time to try and avoid a probable memory leak.
06-Nov-2015 12:29:26.654 SEVERE [localhost-startStop-2] org.apache.catalina.loader.WebappClassLoaderBase.checkThreadLocalMapForLeaks The web application [ROOT] created a ThreadLocal with key of type [java.lang.ThreadLocal] (value [java.lang.ThreadLocal@682d21a6]) and a value of type [quickfix.field.converter.UtcTimestampConverter] (value [quickfix.field.converter.UtcTimestampConverter@6b6c6237]) but failed to remove it when the web application was stopped. Threads are going to be renewed over time to try and avoid a probable memory leak.
06-Nov-2015 12:29:26.654 SEVERE [localhost-startStop-2] org.apache.catalina.loader.WebappClassLoaderBase.checkThreadLocalMapForLeaks The web application [ROOT] created a ThreadLocal with key of type [java.lang.ThreadLocal] (value [java.lang.ThreadLocal@682d21a6]) and a value of type [quickfix.field.converter.UtcTimestampConverter] (value [quickfix.field.converter.UtcTimestampConverter@2aca975d]) but failed to remove it when the web application was stopped. Threads are going to be renewed over time to try and avoid a probable memory leak.
06-Nov-2015 12:29:26.654 SEVERE [localhost-startStop-2] org.apache.catalina.loader.WebappClassLoaderBase.checkThreadLocalMapForLeaks The web application [ROOT] created a ThreadLocal with key of type [java.lang.ThreadLocal] (value [java.lang.ThreadLocal@682d21a6]) and a value of type [quickfix.field.converter.UtcTimestampConverter] (value [quickfix.field.converter.UtcTimestampConverter@2bebb3b3]) but failed to remove it when the web application was stopped. Threads are going to be renewed over time to try and avoid a probable memory leak.
06-Nov-2015 12:29:26.655 SEVERE [localhost-startStop-2] org.apache.catalina.loader.WebappClassLoaderBase.checkThreadLocalMapForLeaks The web application [ROOT] created a ThreadLocal with key of type [java.lang.ThreadLocal] (value [java.lang.ThreadLocal@682d21a6]) and a value of type [quickfix.field.converter.UtcTimestampConverter] (value [quickfix.field.converter.UtcTimestampConverter@4b8bfee1]) but failed to remove it when the web application was stopped. Threads are going to be renewed over time to try and avoid a probable memory leak.
```

It has something to do with `UtcTimestampConverter` but I have no idea why is it happening. I correctly shutdown all threads I manually start.