wpdb: set dbh to null when the mysqli connection fails. see #21663.

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Add a constant to disable mysqli for testing purposes. see #21663.

Built from https://develop.svn.wordpress.org/trunk@27278


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When failing to reconnect to a server that has gone away, simply fail the query once we've passed template_redirect, rather than wp_die(). 

props pento.
see #5932.

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