Additional checks during bucket sampling

[cygnusv]

Bucket sampling can also check for:

• operator funds,
• client version,
• block numbers (this one requires client update).

[KPrasch]

related to nucypher/nucypher#3401

[derekpierre]

Related to https://github.com/nucypher/sprints/issues/36

[cygnusv]

Bumping on the operator funds! A recent DKG almost failed because the last node only had 0.6 POL to submit aggregates, during a high-gas period.