Lazily obtain CQL for exception translation

[samueldlightfoot]

ReactiveCassandraTemplate (and potentially other places) eagerly obtain the statement CQL to be used for exception translation (in the case of an error). This is lightweight for most types of statements except for BatchStatement's which build up the CQL iteratively, looping over each statement. The production of the CQL should be done lazily, only in the case of an error such that the work is not performed in the normal flow.

I've raised an issue for awareness. I'm also happy to pick it up.