RX ring

rx_ring is a circular buffer, where we store packets, grouped by FEC blocks. It has two variables: rx_ring_front (index of the first allocated FEC block) and alloc_size -- number of allocated blocks. So rx_ring is like a queue of FEC blocks (each block can hold up to N fragments) - we append new fragments to block(s) in the tail and fetch them from the head.

When we receive a new packet it can belongs to:

1. New fec block - we need to allocate it in RX ring (do nothing if block was already processed)
2. Already existing fec block - we need to add it to them (do nothing if packet already processed)

If we successfully decode all fragments from the block the we remove ALL unfinished blocks before it.

When we allocate a new block we have following choices:

1. Add a new block to rx ring tail.
2. Override a block at rx ring head if rx ring is full.

So we can support invariant that output UDP packets will be always ordered and no duplicates will be inside.