Use the correct chain var for bloom filter reset

[mawdegroot]

When running with validator challenges the bloom filter is still reset using poc_challenge_interval which is not used with validator challenges. Change to the correct chain variable and use GatewayCount / PocChallengeRate to mimic old bloom filter behavior.

blockchain_ledger_v1:gateway_count is quite slow so adding it to the record to cache the result for the duration of one interval length.

[abhay]

This likely needs some testing but it also needs a guard around rollback scenario if we (unlikely) roll back the light hotspot activation.

[mawdegroot]

From what I understood the miner_poc_mgr is only used with validator challenges, if we roll back to hotspot challenges we will fall back to miner_poc_statem so a guard shouldn't be necessary.

How do I best test this?