Add pg_size_bytes() to parse human-readable size strings.

 Add pg_size_bytes() to parse human-readable size strings.
 
 This will parse strings in the format produced by pg_size_pretty() and
 return sizes in bytes. This allows queries to be written with clauses
 like "pg_total_relation_size(oid) > pg_size_bytes('10 GB')".
 
 Author: Pavel Stehule with various improvements by Vitaly Burovoy
 Discussion: http://www.postgresql.org/message-id/CAFj8pRD-tGoDKnxdYgECzA4On01_uRqPrwF-8LdkSE-6bDHp0w@mail.gmail.com
 Reviewed-by: Vitaly Burovoy, Oleksandr Shulgin, Kyotaro Horiguchi,
     Michael Paquier and Robert Haas