Memory footprint improvements around the FOF code

This was needed to get the big flamingo job going. Changes inspired by !1393 (merged).

Main ideas:

• Free all the foreign buffers when entering FOF
• In FOF only allocate the gpart foreign buffers as we do not need the rest
• Allocate the buffers as late as possible so that the memory can be used for the other FOF-search arrays before that.
• Restore all the buffers as they were when done.