Support snapshot requests without valid GXID

This allows callers to request snapshots without first obtaining a GXID from
the GTM. This should help read-only transactions.

In passing, also make some adjustments to the proxy code and add a check for
messages that are proxied between proxy and GTM. Otherwise a mistmatch between
message proxying and later response processing can cause hard-to-find bugs
(like one we srtuggled while creating this patch)

We still want some more enhancements so that a snapshot once obtained can be
recorded at the GTM so that subsequent requests can get the same snapshot, for
example for serializable transactions

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