Fix a bug in the threadtest4.c program. Remove the keyinfo cache as it provides

Fix a bug in the threadtest4.c program.  Remove the keyinfo cache as it provides
minimal performance improvements, and then only at SQL preparation time, not
at runtime, and it has problems with data races in shared-cache mode.  We might
later add the keyinfo cache back but only enable it when shared-cache mode
is off.

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