libpq async connection and multiple hosts

Hello,

We are aware that, using async connection functions (`PQconnectStart`,
`PQconnectPoll`), the `connect_timeout` parameter is not supported;
this is documented at
https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-PQCONNECTSTARTPARAMS

"""
The connect_timeout connection parameter is ignored when using
PQconnectPoll; it is the application's responsibility to decide
whether an excessive amount of time has elapsed. Otherwise,
PQconnectStart followed by a PQconnectPoll loop is equivalent to
PQconnectdb.
"""

However, ISTM that connecting to multiple hosts is not supported
either. I have a couple of issues I am looking into in psycopg 3:

- https://github.com/psycopg/psycopg/issues/602
- https://github.com/psycopg/psycopg/issues/674

Do we have to reimplement the connection attempts loop too?

Are there other policies that we would need to reimplement? Is
`target_session_attrs` taken care of by PQconnectPoll?

On my box (testing with psql and libpq itself),
PQconnect("host=8.8.8.8") fails after 2m10s. Is this the result of
some unspecified socket connection timeout on my Ubuntu machine?.

If we need to reimplement async connection to "host=X,Y", we will need
to use a timeout even if the user didn't specify one, otherwise we
will never stop the connection attempt to X and move to Y. What
timeout can we specify that will not upset anyone?

Thank you very much

-- Daniele