Bluetooth: Mesh: Slab based segmentation handling

Allocates segmented message buffers as slabs in a common pool for RX and
TX. This reduces memory requirements for both TX and RX, as TX messages
can be stored without the network and advertising buffer overhead, and
RX can use only the slabs it needs, instead of allocating a full size
segmented message. This approach also removes the need for decrypting
the segments for each retransmission, reducing overall processing load.

Slab based segmentation for tx also introduces queuing of segmented
messages, which allows the application layer to send multiple messages
to the same destination without violating Bluetooth Mesh specification
v1.0.1, section 3.6.4.1. This mechanism is provided through a flag that
blocks segmented messages to a destination which a message is already
being sent to until the previous message finishes.

This changes the SDU size configuration to a symmetrical
RX_SEG_MAX/TX_SEG_MAX pair of configurations, plus a new segment pool
side configuration. It also removes the binding between the TX_SEG_MAX
config and the advertising buffers, reducing the minimum advertising
buffer count from 6 to 3.

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