fix: stale dns plugin when connected to reader

[faviansamatha]

Description

Currently, we can run into the following condition:

conn: Connection = connect_func()  # conn is connected to `reader` because dsn hasn't resolved

cluster_inet_address: Optional[str] = None
try:
    cluster_inet_address = socket.gethostbyname(host_info.host) # dsn resolved to writer and now cluster_inet_address is correct
except socket.gaierror:
    pass
            

When this happens, we see that the current connection is a reader; however, we do not switch the connection because the cluster_inet_address == writer_host_address (since it resolves).

This PR adds a check to make sure that if we are connected to a reader, we will attempt to connect to a writer host.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.