MySQL 5.5 dropped support for SQL_TSI_FRAC_SECOND; now using non-standard MICROSECOND

MySQL used to support SQL_TSI_FRAC_SECOND, but (for whatever reason) dropped that in favor of MICROSECOND. Our translator now needs to be able to recognize this new value.

Unless changed, users cannot add fractions of seconds to a timestamp using the timestampadd function. (A small-probability usecase.)