If approval checking falls behind a certain threshold, it means that the
network could not process the assignments and approvals needed for
approving the block fast enough, so we need to back-off from creating
new work, to give the opportunity of the approvals subsystems to catch
up.

Continously, creating new work is not a good idea because of the way
approvals subsystems work, so if the system is slow on processing the assignments
and approvals for the current block, either because we are behind on work
from previous blocks or because the network is slow, validators will
simply trigger new tranches which in turn causes more delays so we are
going to create the conditions for the system to never catch up and fall
behind.

Hence, why we need a mechanism to ensure that instead of falling
more and more behind we actually allow the system to automatically catch up
and start working in optimal conditions. This PR achieves that, by
abstaining from backing new candidates if the node is behind on
approvals beyond a certain threshold.

Signed-off-by: Alexandru Gheorghe <[email protected]>

Signed-off-by: Alexandru Gheorghe <[email protected]>