Consistently use db_to_json to convert from database values to JSON objects.

[clokep]

This removes the last usages of importing json from canonicaljson in order to get the simplejson package.

This consistently uses the db_to_json helper in our storage code. So a follow-on to #7836 / #7802 and related to #7674.

[clokep]

There are other instances of json.loads being used directly in synapse.storage -- should we be converting all of those to db_to_json? It is unclear to me in what situations postgresql returns a memoryview / in what situations we get bytes back instead of strings.

[clokep]

There are other instances of json.loads being used directly in synapse.storage -- should we be converting all of those to db_to_json? It is unclear to me in what situations postgresql returns a memoryview / in what situations we get bytes back instead of strings.

I decided to make this change, see 281d4de29eb776c28ce23d1c72bdeb4f1dc10e0b

[clokep]

I suspect it would be nice to have a json_to_db helper as well that removes whitespace and such as #7372 suggests. Out of scope for this specific PR though.

[richvdh]

hrm. It's unsatisfactory if synapse.storage._base has a dependency on this file somehow. This is fine as a temporary solution though.

[clokep]

synapse.storage._base imports from synapse.storage.database, which imports from synapse.storage.background_updater.