relayd: do not prioritize control events over data.

relayd: do not prioritize control events over data.

Simplify the algorithm used by relayd for control and data connections
handling.

Use the notion of activity phase. An activity phase represent a phase
for which all connections with activity (poll/epoll) are not yet processed.

When an active connection is processed, her activity phase is set to the
current activity phase to prevent further progress during the same
activity phase.

Once all active connections (poll events) have been processed during
the current activity phase, the current activity phase is incremented.

This give fairness across all connections during a given activity phase.

This can also serve as a base for future work toward resources based
prioritizing.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>