Enable the caller of the trigger_dag API endpoint to specify a
boolean value for replace_microseconds.  When false, this will
store microseconds in the database for the execution_id, thus allowing
more than 1 request per second to be processed by the REST API

No need to require a user to pass in replace_microseconds to the
request body; instead we should use exactly the date that is given.
We will still replace the microseconds on execution_date if none is
passed in (and the param is True, which is the default)

A user can now choose to pass the execution_date in but still have
the microseconds replaced by also sending replace_microseconds=true
in the request body

The only way that replace_microseconds should be accessible
is through the endpoints, so leave the testing there