Improve selectivity estimation for assorted match-style operators.

Quite a few matching operators such as JSONB's @> used "contsel" and
"contjoinsel" as their selectivity estimators.  That was a bad idea,
because (a) contsel is only a stub, yielding a fixed default estimate,
and (b) that default is 0.001, meaning we estimate these operators as
five times more selective than equality, which is surely pretty silly.

There's a good model for improving this in ltree's ltreeparentsel():
for any "var OP constant" query, we can try applying the operator
to all of the column's MCV and histogram values, taking the latter
as being a random sample of the non-MCV values.  That code is
actually 100% generic, except for the question of exactly what
default selectivity ought to be plugged in when we don't have stats.

Hence, migrate the guts of ltreeparentsel() into the core code, provide
wrappers "matchingsel" and "matchingjoinsel" with a more-appropriate
default estimate, and use those for the non-geometric operators that
formerly used contsel (mostly JSONB containment operators and tsquery
matching).

Also apply this code to some match-like operators in hstore, ltree, and
pg_trgm, including the former users of ltreeparentsel as well as ones
that improperly used contsel.  Since commit 911e70207 just created new
versions of those extensions that we haven't released yet, we can sneak
this change into those new versions instead of having to create an
additional generation of update scripts.

Patch by me, reviewed by Alexey Bashtanov

Discussion: https://postgr.es/m/12237.1582833074@sss.pgh.pa.us

src/include/utils/selfuncs.h

@@ -42,6 +42,9 @@
 /* default selectivity estimate for pattern-match operators such as LIKE */
 #define DEFAULT_MATCH_SEL	0.005
 
+/* default selectivity estimate for other matching operators */
+#define DEFAULT_MATCHING_SEL	0.010
+
 /* default number of distinct values in a table */
 #define DEFAULT_NUM_DISTINCT  200
 
@@ -148,6 +151,9 @@ extern double histogram_selectivity(VariableStatData *vardata, FmgrInfo *opproc,
 									Datum constval, bool varonleft,
 									int min_hist_size, int n_skip,
 									int *hist_size);
+extern double generic_restriction_selectivity(PlannerInfo *root, Oid operator,
+											  List *args, int varRelid,
+											  double default_selectivity);
 extern double ineq_histogram_selectivity(PlannerInfo *root,
 										 VariableStatData *vardata,
 										 FmgrInfo *opproc, bool isgt, bool iseq,