Add pg_show_plans.skip_queryids guc

[legrandlegrand]

this is to try a new guc: pg_show_plans.skip_queryids,
that contains a list of queryids that won't have their plan displayed.

testing:

do $$ declare i int;j int; begin for i in 1..1000000 loop select 1 into j; end loop; end; $$;

initial configuration:
explain (costs off)
pg_show_plans_enable
pgss.track='all'
pgsp.skip_queryids not set (NULL)

			skip_queryids	NULL	1(*)	10(**)
select 1+1 				2300	2350	2350
select 1				2300	1570	1570
	pgss.track='none'		2040
	pg_show_plans_disable		1220

(*) skip_queryids containing queryid of "select 1" statement.
(**) skip_queryids containing queryid of "select 1" statement + 9 others

Impact on non skipped queries like "select 1+1" stay very low (2350 ms / 2300 ms)