Re-enable small offenders when disabled validators count reaches the disabling limit

[tdimitrov]

With the new disabling strategy in place if the number of disabled validators reaches the Byzantine threshold no more validators are disabled. This potentially opens an attack vector. If a malicious party manages to disable enough honest validators to reach the Byzantine threshold it will have free tries to push bad candidate till the end of the era.

Alternative approach is to re-enable small offenders in favour of big ones when the Byzantine threshold is reached.

This issue is a follow up from #2226 (comment)

[eskimor]

the free tries is correct, but a bit misleading. The attacker would still lose all its stake, but then has nothing to lose and can keep trying for an entire era.

[Overkillus]

As per the recent summary in here. We definitely want to re-enable lowest offenders when reaching the bzt.

The initial mvp will most likely not have this feature enabled and it can be added at the second release.

Some of the reasoning behind it is that it helps us protect against situations where 1/3 of validators are slashed for a low amount and then malicious backers can get away free with endless backing during an era. Re-enabling makes it so endless malicious backing is only possible if at least 1/3 of the total active validator stake is slashed.