Add maxAllowedPacket connection option (send/receive limit)

Today the connector has no client-side control over packet size: the handshake max_packet_size field is hardcoded to 1 GB, and the outgoing/incoming limits are driven only by the server's max_allowed_packet.

Add a maxAllowedPacket connection option that:

• caps the maximum size of packets the driver will send;
• caps the maximum size of packets the driver will accept when reading.

Received packets are size-checked in two phases:

• Before authentication: a fixed 1 Mb cap.
• After authentication:
  • if maxAllowedPacket is set, then that value is honored
  • if unset, then a server-independent heap-relative ceiling: a quarter of the JVM max heap, clamped between 16 Mb and 1 Gb.