Added support for parsing MySQL/Connector boolean and integer
arguments within the URL query string: connection_timeout,
connect_timeout, pool_size, get_warnings,
raise_on_warnings, raw, consume_results, ssl_verify_cert, force_ipv6,
pool_reset_session, compress, allow_local_infile, use_pure.

Change-Id: I2a1a17d13d47d56871bff32e94fdbed8bc003ad7
Fixes: #3787

Fixed regression in JSON datatypes where the "literal processor" for
a JSON index value, that needs to take effect for example within DDL,
would not be invoked for the value.  The native String and Integer
datatypes are now called upon from within the JSONIndexType
and JSONPathType.  This is applied to the generic, Postgresql, and
MySQL JSON types.

Change-Id: Ifa5f2acfeee57a79d01d7fc85d265a37bd27c716
Fixes: #3765

Change-Id: I23a6abb26bbbe3d118887d043ce761fc4572d8d2
Fixes: #3766

this must have been an oversight.   Leave mysql.JSON in place still
as we might need to add things to it.
- CAST(json, String) still confusing MySQL drivers even mysqlclient.
Since here we're checking for the raw form of the JSON and not a typical
round trip, add a force for unicode

Change-Id: I727f4526a01a0875037cf0dede45cd074da5205a

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

Pull request courtesy Stefan Urbanek.  fixes #1957

- make docs for isolation level more consistent between postgresql
and mysql
- move mysql autocommit tests

fixes #3095, #3292
- reorganize enum constructor to again work with the MySQL
ENUM type
- add a new create_constraint flag to Enum to complement that of
Boolean
- reinstate the CHECK constraint tests for enum, these already
fail /skip against the MySQL backend
- simplify lookup rules in Enum, have them apply to all varieties
of Enum equally