lib: Support for rudimentary retransmission

This adds rudimentary support for sending and processing
acknowledgments and doing retransmission.

It replaces the generic timerwheel with a specific one for
retransmission. This is currently a fixed wheel allowing
retransmissions to be scheduled up to about 32 seconds into the
future. It currently has an 8ms resolution. This could be made
configurable in the future. Failures of the flow (i.e. rtx not
working) are indicated by the rxmwheel_move() function returning a fd.
This is currently not yet handled (maybe just setting the state of the
flow to FLOWDOWN is a better solution).

The shm_rdrbuff tracks the number of users of a du_buff. One user is
the full stack, each retransmission will increment the refs counter
(which effectively acts as a semaphore). The refs counter is
decremented when a packet is acked. The du_buff is only allowed to be
removed if there is only one user left (the "stack").

When a packet is retransmitted, it is copied in the rdrbuff. This is
to ensure integrity of the packet when multiple layers do
retransmission and it is passed down the stack again.

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

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