pgsql: Show names of DEALLOCATE as constants in pg_stat_statements

Show names of DEALLOCATE as constants in pg_stat_statements

This commit switches query jumbling so as prepared statement names are
treated as constants in DeallocateStmt.  A boolean field is added to
DeallocateStmt to make a distinction between ALL and named prepared
statements, as "name" was used to make this difference before, NULL
meaning DEALLOCATE ALL.

Prior to this commit, DEALLOCATE was not tracked in pg_stat_statements,
for the reason that it was not possible to treat its name parameter as a
constant.  Now that query jumbling applies to all the utility nodes,
this reason does not apply anymore.

Like 638d42a3c520, this can be a huge advantage for monitoring where
prepared statement names are randomly generated, preventing bloat in
pg_stat_statements.  A couple of tests are added to track the new
behavior.

Author: Dagfinn Ilmari Mannsåker, Michael Paquier
Reviewed-by: Julien Rouhaud
Discussion: https://postgr.es/m/ZMhT9kNtJJsHw6jK@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/bb45156f342c2cdb88cdd7f9efdc4541e910ec86

Modified Files
--------------
contrib/pg_stat_statements/expected/utility.out | 41 +++++++++++++++++++++++++
contrib/pg_stat_statements/pg_stat_statements.c |  8 ++---
contrib/pg_stat_statements/sql/utility.sql      | 13 ++++++++
src/backend/parser/gram.y                       |  8 +++++
src/include/nodes/parsenodes.h                  |  8 +++--
5 files changed, 70 insertions(+), 8 deletions(-)