* PG-606: New GUC required for enabling/disabling of pgsm_query_id calculation
Adds a new GUC pg_stat_monitor.pgsm_enable_pgsm_query_id to enable/disable
pgsm query id calculation. Apart from that patch also refactors the GUC-related
code to match PostgreSQL conventions.
Moreover, the commit also changes the pgsm_enable_overflow GUC to boolean
instead of enum.