Emit SET NAMES for all MySQL connections w charset

Emit SET NAMES for all MySQL connections w charset

The MySQL dialects will emit "SET NAMES" at the start of a connection when
charset is given to the MySQL driver, to appease an apparent behavior
observed in MySQL 8.0 that raises a collation error when a UNION includes
string columns unioned against columns of the form CAST(NULL AS CHAR(..)),
which is what SQLAlchemy's polymorphic_union function does.   The issue
seems to have affected PyMySQL for at least a year, however has recently
appeared as of mysqlclient 1.4.4 based on changes in how this DBAPI creates
a connection.  As the presence of this directive impacts three separate
MySQL charset settings which each have intricate effects based on their
presense,  SQLAlchemy will now emit the directive on new connections to
ensure correct behavior.

Fixes: #4804
Change-Id: If9d7ee00d0ccaf773972b564fe455e8e9edf6627