Consider preferencing more full lists when choosing a new block

[dlongley]

For current behavior, see: https://github.com/digitalbazaar/bedrock-vc-issuer/blob/v30.0.2/lib/IndexAllocationCache.js#L289-L293

We might, instead, want to preference block assignment docs that have fewer available blocks so as to "exhaust" earlier lists. Privacy impact analysis required.