Vote: BlockOperator interface

[tzanio]

There has been a series of issues around the const-ness of the elements of BlockOperator: #4078, #4135 and #4136. Lets vote below on how to proceed.

[najlkin]

Maybe a certain compromise solution, which I do not like much 😕 , but just offering 😉. We could introduce TBlockOperator internally as an implementation of Block(Const)Operator (avoiding duplicity of the code) without exposing the template to users if we do not trust them to use it correctly 🤔 .

[ebchin]

Maybe there should be an option "Revert the const changes introduced in #4078 and merge #4135"?

[tzanio]

Sorry, the second option should not have been in the poll -- #4135 is orthogonal to the two choices and we probably want to merge it either way.