Stopped stripping microseconds with MySQL backend

Refs #19716.
"The MySQL backend no longer strips microseconds  from datetime values as MySQL 5.6.4 and up supports fractional seconds depending on the declaration of the datetime field."

re: "depending on the declaration of the datetime field" -- some clarification would be good. Are Django's built in fields defined appropriately, or is this only relevant for custom fields? Does MySQL issue any warnings when discarding the microseconds?