Introduce squashing of constant lists in query jumbling

Introduce squashing of constant lists in query jumbling

pg_stat_statements produces multiple entries for queries like
    SELECT something FROM table WHERE col IN (1, 2, 3, ...)

depending on the number of parameters, because every element of
ArrayExpr is individually jumbled.  Most of the time that's undesirable,
especially if the list becomes too large.

Fix this by introducing a new GUC query_id_squash_values which modifies
the node jumbling code to only consider the first and last element of a
list of constants, rather than each list element individually.  This
affects both the query_id generated by query jumbling, as well as
pg_stat_statements query normalization so that it suppresses printing of
the individual elements of such a list.

The default value is off, meaning the previous behavior is maintained.

Author: Dmitry Dolgov <[email protected]>
Reviewed-by: Sergey Dudoladov (mysterious, off-list)
Reviewed-by: David Geier 
Reviewed-by: Robert Haas 
Reviewed-by: Álvaro Herrera 
Reviewed-by: Sami Imseih 
Reviewed-by: Sutou Kouhei 
Reviewed-by: Tom Lane 
Reviewed-by: Michael Paquier 
Reviewed-by: Marcos Pegoraro 
Reviewed-by: Julien Rouhaud 
Reviewed-by: Zhihong Yu 
Tested-by: Yasuo Honda 
Tested-by: Sergei Kornilov 
Tested-by: Maciek Sakrejda 
Tested-by: Chengxi Sun 
Tested-by: Jakub Wartak 
Discussion: https://postgr.es/m/CA+q6zcWtUbT_Sxj0V6HY6EZ89uv5wuG5aefpe_9n0Jr3VwntFg@mail.gmail.com