fix bad-caching of datetime with tz info at sql generation time There is a special handling for datetime with tzinfo, where value was stored in the query cache. The implementation of merge_args was simply overwriting parameters of the query with those in the query cache, expecting no collision. To fix this: * handle replacement of tzinfo in merge_args, not at sql generation time * add an assertion to ensure we've actually no collision Closes #6978316

# re-read ComputedRelation permissions from schema.py now that we're
# able to serialize them
for computedrtype in schema.iter_computed_relations():
    sync_schema_props_perms(computedrtype.type)