Multiple LSN support for single connection

[karolisg]

At the moment we support single tuple of (lsn, seq_no). It works fine when connector has one lsn for all tables. Altough, it doesn't work for snowflake, because there we have independent table streams and with single lsn tuple we cannot determine where to resume replication. @chubei has suggested this approach

We can change the message identifier definition from per source to per source per output port, so we can still use one ConnectorSource but output independent message identifiers to different output ports and Source::start_from receives HashMap<PortHandle, Option<(u64, u64)> instead of Option<u64, u64>. Postgres connector is just a special case who always output same identifier to all ports, which the pipeline don't need to handle.

@snork-alt @v3g42 What do you think about such change?

[v3g42]

As discussed, makes sense to initialize separate connectors in my view instead of changing the connector interface.

Maybe we can think of a way to share connector pool as a separate item ?