Do we need a separate `writer` pool?

[vincentsarago]

          @gadomski @hrodmn I'm not going to change the `PostgresSettings` class in this PR. 

I assume all user already set the postgres_host_writer variable anyway

My knowledge in postgres is limited but I'm having hard time to understand how can the writer and reader host be different cc @bitner

Originally posted by @vincentsarago in #236 (comment)

[bitner]

They are different in the case that you may have a Read/Write Database that needs to handle any writes, but you may have Read Replicas that you use for normal read activity. Or if you want to really make sure that you are using "least privilege possible" for any operations against the db. For most cases that I've seen, they'll just be the same though.

[vincentsarago]

@bitner but would those only have a host different? I feel that they would also have user/password being different

[bitner]

Yes, all connection parameters would potentially be different