Fill DAL slots with more than one chunk in rollup node DAL injector

This MR extends the DAL injection queue to concatenate the maximum of chunks (while complying with the ordering of their addition in the injection queue) before publishing a DAL slot.

Note. The tezt test seems flaky. It might be put in a separate MR to be able to block the logic (which is quite trivial)

Test moved to !14913 (merged)