scoping the message_store schema with security definer

[lgazo]

This was my attempt to fix the problem of scoping to the message_store and not to pollute the schema in every query. I am not PG expert so not sure if this path is the correct one. In general, the question is whether we need to scope it at all. But in case this sounds like a good solution to the problem #43 / #49 then feel free to accept it. I will also probably try to remove the schema prefix and rather limit it with the privileges for particular user.