OP Guide: Restructuring some sections / chapters

Pulled unrelated sections out of the "L2 address masking" chapter.

They now have their own chapters:
* Handling fragmented packets
* MidoNet resource protection

Change-Id: I7ae19b1ff7c61bb353cc83a20a4977a54c2e90fe
Signed-off-by: Jan Hilberath <jan@midokura.com>