use a full ConnectionCharacteristic for logging_token

use a full ConnectionCharacteristic for logging_token

Fixed issue in the
:paramref:`_engine.Connection.execution_options.logging_token` option,
where changing the value of ``logging_token`` on a connection that has
already logged messages would not be updated to reflect the new logging
token.  This in particular prevented the use of
:meth:`_orm.Session.connection` to change the option on the connection,
since the BEGIN logging message would already have been emitted.

Fixes: #11210
Change-Id: I0ddade3778215259a6eacde3a67e09d30bc3257b