Bypass normalization of already prepared statements.

[aquarapid]

• The normalization we do in vtgate during planning before checking the plan cache is expensive for large statements (think multi-value inserts).
• For statements that are already prepared, normalizing them adds little value, so we should bypass that.

The only thing to consider here is whether we should make this configurable behavior.

For a bit of performance context, on a workload where prepared statements are used to insert 100 rows into a 40-way sharded table at a time, this chops the plan time for each query in half in my test environment (from ~ 4 ms per query to < 2 ms per query).

[aquarapid]

As context; the use-case here are large inserts into MySQL backend(s) that are pretty speedy (running on bare metal), and the actual overall time spent per insert is 8-10 ms; so the plan time ends up being almost 50% of that...

[aquarapid]

For release notes:

In previous Vitess releases, prepared statement queries were normalized by vtgate if
-normalize_queries is true (which it is by default).  We have introduced the new
vtgate flag -normalize_prepared_stmts which defaults to false.  This changes the
default behavior of vtgate to *not* normalize prepared statements, but still
normalize non-prepared statement queries.  This can increase performance
if your workload is prepared statement heavy, by reducing the CPU overhead
in vtgate, and by reducing the planning latency, especially for large queries. You
can revert to the previous vtgate behavior by setting the -normalize_prepared_stmts
flag to true.

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[maxenglander]

@vmg I think I addressed the last outstanding comment. Let me know if there is anything else I should address. I saw @aquarapid mention above about release notes. Should I add those here as part of this PR?

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