[migration] fix change_attribute_type to update the live schema Closes #7170830

[devtools] add has_cache for postgres (closes #5739624) devtools stores info about existing dbs in the db handler, but in the case of postgresql that doesn't take into account the path to the cluster's datadir. Which means if we run two test modules (in the same test run), we'll create a "__default_empty_db__" for the first one, cache its existence, and then when moving on to the other module, believe the template already exists (but since the datadir depends on the test module's path, it does not). This patch is a bit of a kludge, and it would be better to make the cache key include enough data to not need this, but I'm not sure how to do that.

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

Added tag 3.20.10, debian/3.20.10-1, centos/3.20.10-1 for changeset 8f82e9523962