Bluetooth: Introduce separate pool for discardable events

Introduce a separate buffer pool for events which the HCI driver
considers discardable. Examples of such events could be e.g.
Advertising Reports. The benefit of having such a pool means that the
if there is a heavy inflow of such events it will not cause the
allocation for other critical events to block and may even eliminate
deadlocks in some cases.

Also update all mesh samples not to specify explicit RX buffer counts
anymore. Instead, create appropriate defaults in Kconfig so that we
only need to override this in the app for cases like the bbc:microbit
with limited memory.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>