Get all live MySQL tests passing.

Required just a few additional minor changes:

* The MySQL docs actually recommend using a huge number to fetch search
  results from an offset with no limit. What*ever*.
* Fix the call to the `group_concat()` aggregate to actually do
  space-delimited concatenation.
* Table-qualify a couple of additional references to the column named
  "change".
* Add a function to the test to add a second to a datetime. This is because,
  like Oracle, the UTC_TIMESTAMP() function returns the statement time, not
  wallclock time.
Fix tag array sort order on MySQL.

The other databases default to using the row order for the agrregation order.
MySQL's `group_concat()`, alas, is random. One can specify an `ORDER BY`
clause for it, but since tags for a single change are inserted at the same
time, they have the same `committed_at` value, so that doesn't work. So we
just have the test force the order on MySQL and don't worry about it.

Also, set `group_concat_max_len` to 32768. I assume that should be enough
characters to represent most lists of things.