The patch works for me (even if there is still a small freeze during
about 1 second at the connection if the server doesn't answer), but
creates an issue with the auto reconnect feature.

It happens when the server doesn't answer the connection request (tested
with a DROP on packets coming from it with iptables).

The patch makes the function erc-server-connect to return even if the
connection is not ready. Then erc-open and erc-server-reconnect do the
same.

Finally, in erc-process-sentinel-2, there is no error executing
erc-server-reconnect, thus erc-server-reconnect-count is never
incremented and is even reset.

I made a patch that increments erc-server-reconnect-count before calling
erc-server-reconnect instead, and resets it only when the connection is
really ready, that is, in erc-connection-established.