dbquery: fix dbexec CLI WRT max allowed packet
dbquery: fix dbexec CLI WRT max allowed packet

* Due to MySQL bug <http://bugs.mysql.com/bug.php?id=41486>,
  this commit introduces --max_allowed_packet=1G in dbexec.in.

inveniosoftware/invenio,tiborsimko/invenio,tiborsimko/invenio,inveniosoftware/invenio