default maxAllowedPacket value when not set

MitM server can drive client memory to exhaustion by streaming endless 0xffffff multi-part fragments: reassembly in PacketInputStream grows unbounded.

CONJS-358 closed the pre-authentication window only, and did so after buffering a full 16Mb fragment.

Changes:
When unset, maxAllowedPacket resolves to a quarter of the memory available to the process: the cgroup limit when containerised, physical memory otherwise, bounded to [16Mb, 1Gb].