Optimize packet allocations

Instead of using QSharedPointer, use std::unique_ptr to reference IncomingPacket. Also, allocate IncomingPacket structure and the packet data in a single memory allocation.

Similarly, replace QSharedPointer with boost::intrusive_ptr to reference Packet and allocate packet data in a single allocation with Packet.

This reduces the number of allocations per packet from 3 (QSharedPointer control block, IncomingPacket/Packet struct and packet data) to just one. Additionally, for IncomingPacket, this removes atomic reference counting implemented by QSharedPointer.

Lastly, avoid passing Packet::Ptr to PacketSocket::preProcess to skip atomic reference count increment/decrement.