As per the discussion in #20288, I'd like to deprecate DbalSessionHandler for the following reasons:

• Why not just use PdoSessionHandler instead as DbalSessionHandler is "slower";
• As everything is abstracted, DbalSessionHandler is not more "convenient" than PdoSessionHandler;
• DbalSessionHandler is more "complex" to configure than PdoSessionHandler as you should ensure that it uses a different connection than the one used by the ORM;
• DbalSessionHandler is not really configurable easily in terms of column names (not that this is really something anyone should do) as there is a schema that helps creating the table (which is by the way the only "advantage" of this handler).

WDYT?