Add max_allowed_packet to my.cnf template

Previously, max_allowed_packet was hardcoded to '16M', which is a fine
default, but we needed to change it and weren't able to.  Added it (with
that 16M default) to the params file, and as a parameter to init and
config.

We tested this without passing anything in (no changes) and via a data binding to change it to '64M'.