lib: Complete retransmission logic

This completes the retransmission (automated repeat-request, ARQ)
logic, sending (delayed) ACK messages when needed.

On deallocation, flows will ACK try to retransmit any remaining
unacknowledged messages (unless the FRCTFLINGER flag is turned off;
this is on by default). Applications can safely shut down as soon as
everything is ACK'd (i.e. the current Delta-t run is done). The
activity timeout is now passed to the IPCP for it to sleep before
completing deallocation (and releasing the flow_id). That should be
moved to the IRMd in due time.

The timerwheel is revised to be multi-level to reduce memory
consumption. The resolution bumps by a factor of 1 << RXMQ_BUMP (16)
and each level has RXMQ_SLOTS (1 << 8) slots.  The lowest level has a
resolution of (1 << RXMQ_RES) (20) ns, which is roughly a
millisecond. Currently, 3 levels are defined, so the largest delay we
can schedule at each level is:

Level 0: 256ms
Level 1: 4s
Level 2: about a minute.

Signed-off-by: Dimitri Staessens <[email protected]>
Signed-off-by: Sander Vrijders <[email protected]>

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