MEDIUM: dynbuf: make the buffer_wq an array of list heads

MEDIUM: dynbuf: make the buffer_wq an array of list heads

Let's turn the buffer_wq into an array of 4 list heads. These are chosen
by criticality. The DB_CRIT_TO_QUEUE() macro maps each criticality level
into one of these 4 queues. The goal here clearly is to make it possible
to wake up the most critical queues in priority in order to let some tasks
finish their job and release buffers that others can use.

In order to avoid having to look up all queues, a bit map indicates which
queues are in use, which also allows to avoid looping in the most common
case where queues are empty..