Fix results for cassandra backend in detailed mode

When using the Cassandra backend in detailed mode, attempting to retrieve the results raises an error because of improperly decoded metadata (introduced by 6d64d7a24f15311a0bd13baf7f6a9879176d2d08).

The metadata for `children` and `traceback` are encoded before being grouped into the dictionary, which itself is encoded later prior to being stored in Cassandra. During retrieval in detailed mode, these are not decoded again, causing them to be interpreted as encoded unicode literals instead of their native types.

CHANGES:
- the `results` metadata is encoded regardless of whether Cassandra is used in detailed or non-detailed mode. This is being consistent with the other backends where the results are always encoded. It may be
  unnecessary (results in double encoding), however.

- The logic for encoding/decoding metadata is now consistent for both detailed and non-detailed modes.

REFERENCE:
- 21cdf82fc4310320c21687cf80a3f98802565bd5 - cassandra detailed mode
- 6d64d7a24f15311a0bd13baf7f6a9879176d2d08 - backends support children