More file reassembly work.

 - The reassembly behavior can be modified per-file by enabling or
   disabling the reassembler and/or modifying the size of the reassembly
   buffer.

 - Changed the file extraction analyzer to use the stream to avoid
   issues with the chunk based approach not immediately triggering
   the file_new event due to mime-type detection delay.  Early chunks
   frequently ended up lost before.

 - Generally things are working now and I'd consider this in testing.