Improve SimpleDateFormat instantiation in TupleBuilder

The `TupleBuilder.tuple()` call is very expensive. 
It can affect Spring Integration flow throughput if called for each message.
This pull request replaces the `SimpleDateFormat` instantiation with static `ThreadLocal` variable.
Also, if you're eagerly forcing the creation of the TL, then there is no need in using `initialValue()`. Might as well create the TL, and then `set()` IMHO.