Do not automatically query max_allowed_packet

[julienschmidt]

As a followup to #489, we should consider to use a sensible default value instead of querying max_allowed_packet on every connect, which adds extra roundtrips and thus overhead.
It might also cause problems with proxies etc. (see #489).

See also https://dev.mysql.com/doc/refman/5.5/en/packet-too-large.html