support for ms datetimeoffset datatype

Dialed back the "order the primary key columns per auto-increment"
described in :ref:`change_mysql_3216` a bit, so that if the
:class:`.PrimaryKeyConstraint` is explicitly defined, the order
of columns is maintained exactly, allowing control of this behavior
when necessary.

Change-Id: I9e7902c57a96c15968a6abf53e319acf15680da0
Fixes: #3726

Rolled back the validation rules a bit in :class:`.Enum` to allow
unknown string values to pass through, unless the flag
``validate_string=True`` is passed to the Enum; any other kind of object is
still of course rejected.  While the immediate use
is to allow comparisons to enums with LIKE, the fact that this
use exists indicates there may be more unknown-string-comparsion use
cases than we expected, which hints that perhaps there are some
unknown string-INSERT cases too.

Change-Id: I7d1d79b374a7d47966d410998f77cd19294ab7b0
Fixes: #3725

Change-Id: I8815a68dcf6e5d27f3f82b02f939f72d9a412337

Change-Id: I5ac6ba8234873fdd19bee6c1decd311a9cb4d55e

Change-Id: Ida3ca92b496a1e3ebd761c100201d3660460c2bf