`data_format.serialise` is now used for `mbGet`, and RSM/MAM values are not transtyped to
strings anymore. A serialised dict is now used, items are put in the `items` key.
Comments handling has been refactored to use a list for the potentially multiple comments
nodes.
`rsm` data are now in a `rsm` key of the dict, and `mam` data are merged with other
metadata.
e03eb5bfcc