[vtadmin] Exclude keyspaces from schema cache

[ajm188]

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• TODO

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[falun]

Going to push back this -- it adds a bit of complexity for an unclear use case. Even original requestor was intending to exclude all keyspaces.

My suggestion if we want to go forward with something like this would be to add a noop cache that either never stores or (iirc it may be difficult to plumb that and keep things still working) treats all cache hit as if at is expired.

That said once original requestor discovered DefaultExpiration flag it was decided that that was sufficient and switched over to using it on a 5m TTL. We did get confirmation that the proactive refresh in thread works as well.

All told I think the complexity trade off + workable/better existing solutions means we should drop this as a potential path forward.

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