clear warnings better and separate warning count from actual warnings

[jycor]

This PR cleans up the logic surrounding warnings and clearing them.
The important part was separating the number of "new" warnings from the list of warnings themselves.

Every query should clear out the warnings from the previous query. The exception is show warnings, which only clears the "count" of warnings.

When a server runs a query that produces a warning, it immediately calls show warnings. Since the show warnings query itself should NOT clear the warnings, the warning count would always be > 0, and incorrectly indicate that show warnings produced a warning. This causes an infinite loop in .NET. Now, we always clear the warning count, and only clear the warnings themselves when the query is not show warnings.

We've also had this weird problem of having to double clear warnings, which this should address.

fixes:

• Connector/NET: A warning causes stack overflow dolt#8440
• warnings are not clearing from running the same queries multiple times dolt#6921

[zachmu]

LGTM