STEP_EXECUTION table is now updated after every chunk
StaxEventItemWriter throws informative error for non-existing Resource, and only does it on open (so it will work with MultResourceItemReader)
In a multi-threaded step, the lock that is taken while the step executio nis being updated cannot be inadvertently released by another thread if it fails
Improvements
StaxEventItemWriter#endDocument(..) can be overriden (allows users to fix problems with Woodstox end tag auto-insertions)
