fix PageView migrator spec on mysql

 refs CNVS-7032

 * be prepared for timestamps that are already timestamps
 * handle no sub-second precision in our batch size assumption

Change-Id: Ic286750a275acec28c60feb145733e9e41cbc12a
Reviewed-on: https://gerrit.instructure.com/29499
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>