Provide DSM segment to ExecXXXInitializeWorker functions.

Previously, executor nodes running in parallel worker processes didn't
have access to the dsm_segment object used for parallel execution.  In
order to support resource management based on DSM segment lifetime,
they need that.  So create a ParallelWorkerContext object to hold it
and pass it to all InitializeWorker functions.

Author: Thomas Munro
Reviewed-By: Andres Freund
Discussion: https://postgr.es/m/CAEepm=2W=cOkiZxcg6qiFQP-dHUe09aqTrEMM7yJDrHMhDv_RA@mail.gmail.com