JBAS-837 sqitch revert sanity checks

JBAS-837 sqitch revert sanity checks

1. Revert only the most recent change during deployment failures by
   default (sqitch deploy --mode change).  This can be overridden at the
   command line.

2. Clear the contents of revert/kcls-2.4-base-prod.sql so that under no
   circumstances can the base schema be reverted.

3. Document revert and verify scripts to indicate that the contents are
   intentially empty for pre-sqitch deployments.

Signed-off-by: Bill Erickson <berickxx@gmail.com>