* This version is still sub-optimal and transfers more data amongst the
  I/O server processes than strictly necessary.

* Every server now serves a subset of the clients only.

* This will enable functions in pio_interface later to be written
  without regard for header specifics.

* This allows to have the offset entry available for all types.

* Said generic can be replaced with an MPI-specific version, thus
  cleanly separating serialization of meta-data itself and MPI-
  communication.

* This way all header elements can later be accessed directly.

* This will later ease distinguishing both in the MPI window index.

* It was MPI_Comm * before which was confusing in some calls.