Unnecessary Trailer and Header object allocation for every Message instance

In quickfix.Message, the field initialisers for 'header' and 'trailer' each new up an object.

Generated Message classes then replace these. This creates unnecessary work for the GC.

I see no reason that subclasses need to re-assign the trailer.

The Header can vary with each FIX version, so perhaps the API could be modified such that the header is created by a protected method which is overridden in generated subclasses.