Verify that MySQL thread_stack is >=192k

CiviCRM 4.3 makes heavier use of triggers/stored procs/functions.

During the Toronto sprint, we encountered an issue where a participant had configured MAMP's MySQL using the 'my-small.cnf' configuration which sets thread_stack=128k (which is smaller than the default 256k). This caused attempts to insert new civicrm_contact rows to fail.

The upgrader and installer should verify that thread_stack is reasonably large. As a point of reference, my devbox uses 192k, and the MySQL default is 256k.