tortoise: continue to verify layers despite long period of emptiness

[countvonzero]

Description

see scenario described in #3381

two concerns are raised

• tortoise doesn't receive updates for longer than zdist so it won't compute votes properly and will vote abstain
• lookback window should be around 2000 eventually. and if there were 2000 layers without ballots - then we will evict all the stuff

there may be others. this require testing to verify.