Get about a quarter of the MySQL live tests passing.

The changes to get this far include:

* The string for the `sql_mode` setting must have no spaces, only commas,
  between its values.
* Convert timestamps to UTC and send the value to the database in plain
  iso-8601 format (that is, without a time zone specification).
* Use `current_timestamp` for the default timestamp value.
* Make sure an error value exists before checking it.
* Make sure that LIMIT and OFFSET are on a single line (WTF MySQL?)
* Table-qualify all column names where "change" is included in the list of
  columns. This is becuase "change" is reserved by MySQL, but need not be
  quoted when it is table-qualified.
* Quote "change" in `INSERT INTO` statements, because MySQL is too dumb to
  realize that column names are unambiguously connected to a table name in
  that context. *Eye role*.
* Use the same Sqitch schema name (database) as is used in the other engine
  tests.
* Properly check MySQL error strings (not sure if it will bump into localized
  variations, though).

Next up, dealing with the fact that MySQL discards sub-second precision in its
timestamp values. (*Groan*)