Bluetooth: Mesh: Network loopback

Changes the local network interface to exclusively handle packets for
the local interface, duplicating the buffers in the process.

The loopback mechanism now operates its own packet pool for the local
interface queue. The loopback is moved ahead of encryption, allowing the
local interface packets to go back up the stack without network crypto,
saving a full round of encrypt/decrypt for self-send.

Packets for group addresses the local node subscribes to are now
duplicated, with one unencrypted variant going into the network
queue, and the network bound packets following the regular path to the
advertiser.

Introduces one new configuration for setting the number of loopback
buffers.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>