Verify if mapByPage is useful to handle LazyPages mappings

[ivanopagano]

This piece of handling might be redundant, or could be re-used.

Indeed, to me it looked like a missing piece and without it the other felt very ad-hoc... but in the end we could remove it.

Actually I'd need some time to check if we could put it to use in some place already, but possibly in a separate task work. I would make a separate PR to use it or remove it, based on the outcome of a quick survey

Originally posted by @ivanopagano in https://github.com/Cryptonomic/Conseil/pull/925#discussion_r498043565