Return a copy from the memstore to avoid data races

[Nocccer]

Description

To avoid data races that occured in #703, a copy is now returned by the memstore instead a pointer to the packet.

• Extend the ControlPacket interface with Copy() ControlPacket function.
• Add tests to check if the copy does not share any pointer with the original packet.
• Use the Copy() function in the memstore to return a copy.
• Test changes against the test from Data race while reconnecting #703 that triggered the race condition.

Note: This is my private github account, so don't get confused.