Balance: new option: don't disable last link that is "up" due to Health check failed

Situation: Balance with 2 or 3 WAN links
WAN #1: high bandwidth (WAN link used for most traffic)
WAN #2: lower bandwidth (WAN link mainly for backup)

As soon as the health check for #1 fails, the bulk of traffic will flow over WAN #2, where as a result of all that traffic the health check packets will experience packet loss, as a result the health is determined as “failed”.

Expected: WAN #2, should not be disabled under high loads.

Feature name: don’t disable last WAN link “up” for “Health check failed”.
A.k.a. disable health check when only 1 WAN link is active. This name is not good, because the health check should continue to run on the failing WAN links to enable these WAN links as soon as they return back in “up” state.
A.k.a. ignore failing health checks when WAN link reaches … (f.e. 90%) of configured capacity.

It doesn’t matter whether it is WAN1, WAN2 or WAN3 that is the last link that is up. Health checking just shouldn’t disable the last WAN link (in high traffic situations).