Introduce BYTES unit for GUCs.

This is already useful for track_activity_query_size, and will further
be used in a later commit making the WAL segment size configurable.

Author: Beena Emerson
Reviewed-By: Andres Freund
Discussion: https://postgr.es/m/CAOG9ApEu8bXVwBxkOO9J7ZpM76TASK_vFMEEiCEjwhMmSLiaqQ@mail.gmail.com