Remove circular buffer

[fujiapple852]

The Tracer uses a circular buffer to track the state of all Probe in-flight. To access a Probe in this buffer, the index is calculated as sequence number of the probe modulo the buffer size.

Whilst this works, it has the draw back that the probes for a given round may wrap around the buffer and therefore must be processed with an Iterator rather than a slice.

This changes the logic to index into the buffer using sequence - round_sequence (i.e. the sequence number - the initial sequence number of the round).

Therefore the buffer access will always start from 0 in every round and will never wrap around as the buffer size is the same as the maximum ttl allowed.