TimestampFilter should serialize start and end as longs in the IteratorSetting

Although the TimestampFilter supports using longs to set the start or end timestamp, it formats them as strings using SimpleDateFormat when storing or retrieving them in the IteratorSetting.

This results in exceptions when the timestamps being used aren't able to be formatted as yyyyMMddHHmmssz. For example, try setEnd(253402300800001,true)

Instead, setStart() and setEnd() could just as easily use String.valueOf(long i) to store the values, and init() could retrieve them using Long.valueOf(String s).