pgsql: Add pg_analyze_and_rewrite_varparams()

Add pg_analyze_and_rewrite_varparams()

This new function extracts common code from PrepareQuery() and
exec_parse_message(). It is then exactly analogous to the existing
pg_analyze_and_rewrite_fixedparams() and
pg_analyze_and_rewrite_withcb().

To unify these two code paths, this makes PrepareQuery() now subject
to log_parser_stats. Also, both paths now invoke
TRACE_POSTGRESQL_QUERY_REWRITE_START(). PrepareQuery() no longer
checks whether a utility statement was specified. The grammar doesn't
allow that anyway, and exec_parse_message() supports it, so
restricting it doesn't seem necessary.

This also adds QueryEnvironment support to the *varparams functions,
for consistency with its cousins, even though it is not used right
now.

Reviewed-by: Nathan Bossart <bossartn(at)amazon(dot)com>
Discussion: https://www.postgresql.org/message-id/flat/c67ce276-52b4-0239-dc0e-39875bf81840(at)enterprisedb(dot)com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/25751f54b8e02a8fff62e9dbdbc9f2efbb4e8dc1

Modified Files
--------------
src/backend/commands/prepare.c | 43 ++-----------------
src/backend/parser/analyze.c | 5 ++-
src/backend/tcop/postgres.c | 95 ++++++++++++++++++++++++++++--------------
src/include/parser/analyze.h | 2 +-
src/include/tcop/tcopprot.h | 5 +++
5 files changed, 78 insertions(+), 72 deletions(-)