Fix some issues with tracking nesting level in pg_stat_statements.

Fix some issues with tracking nesting level in pg_stat_statements.

When we decide that we don't want to track execution time of a
specific planner or ProcessUtility call, we still have to increment
the nesting depth, or we'll make the wrong determination of whether
we are at top level when considering nested statements.  (PREPARE
and EXECUTE are exceptions, for reasons explained in the code.)

Counting planner nesting depth separately from executor nesting depth
was a mistake: it causes us to make the wrong determination of whether
we are at top level when considering nested statements that get
executed during planning (as a result of constant-folding of
functions, for example).  Merge those counters into one.

In passing, get rid of the PGSS_HANDLED_UTILITY macro in favor of
explicitly listing statement types.  It seems somewhat coincidental
that PREPARE and EXECUTE are handled alike in each of the places where
that was used: the reasoning tends to be different for each one.
Thus, the macro seems as likely to encourage future bugs as prevent
them, since it's quite unclear whether any future statement type that
might need special-casing here would also need the same choices at
each spot.

Sergei Kornilov, Julien Rouhaud, and Tom Lane, per bug #17552 from
Maxim Boguk.  This is pretty clearly a bug fix, but it's also a
behavioral change that might surprise somebody, so no back-.

Discussion: https://postgr.es/m/17552-213b534c56ab5d02@postgresql.org