Add backend type to csvlog and optionally log_line_prefix

Add backend type to csvlog and optionally log_line_prefix

The backend type, which corresponds to what
pg_stat_activity.backend_type shows, is added as a column to the
csvlog and can optionally be added to log_line_prefix using the new %b
placeholder.

Reviewed-by: Julien Rouhaud 
Reviewed-by: Kuntal Ghosh 
Reviewed-by: Alvaro Herrera 
Reviewed-by: Justin Pryzby 
Discussion: https://www.postgresql.org/message-id/flat/c65e5196-4f04-4ead-9353-6088c19615a3@2ndquadrant.com