Improvement of implementation:
1. Small cleanups
2. improve pmix_io_engine: now message headers may be preprocessed at
receive and before send. This provides ability to convert from host to
network byte order etc.
3. Implement dumb PMIx client. It is similar to PMIx lib in it's
patterns but doesn't use dss for packing. It may be used to tun OMPI
applications with PMIx plugin (temporal pmix module needs to be
implemented). This might be done to test PMIx plugin in real environment
if original pmixlib won't be ready at that time.