Fix conflicting Redis updates implementation (#625)

[ikavalio]

I've tried to fix #625 naively by storing metadata in redis as a hash rather than serialized JSON object (also added scalar implementation path since set_meta / get_meta interface accepts t.Any). Both old format (scalar) and hash are supported for backwards compatibility, but data is always written in the new format.

I tested it locally and new key format seems to be correct:

127.0.0.1:6379> hgetall "queue:db_table:meta"
1) "checkpoint"
2) "80340"
3) "txid_current"
4) "80346"