plugins XEP-0059, XEP-0060, XEP-0277, XEP-0313: better serialisation: `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.