Connect to several hosts concurrently to reduce connect lantency

[vlsi]

Currently, pgjdbc attempts hosts in order (e.g. in strict order or in shuffle order). It does not work well in case there is one host that does not respond.

It would probably make sense to try concurrent connections if the given host did not respond within a configured time.

For instance, consider we have db1,db2,db3 in connection URL.

• Try connecting to db1
• If db1 did not respond within 500ms, start concurrent connections to db2 and db3
• Return the first workable connection to the end user, and close the others

[sehrope]

The SQL Server driver does something similar to this to open sockets in parallel to all resolving IPs for the database host: https://github.com/microsoft/mssql-jdbc/blob/c073e78cd18153b305c29533fe53b02200edf796/src/main/java/com/microsoft/sqlserver/jdbc/IOBuffer.java#L2470-L2602

For the record, I think it's needlessly complicated, non-deterministic, and it's had more than one bug in the past so let's tread carefully. If we do add something like this, is should be opt in as defaulting to creating threadpools or parallel connection is generally a bad idea.

Also, without opt-in, anybody upgrading with multiple resolved hosts would suddenly see connection errors on their server logs for all the "slow" connections as they'd be closed after socket creation but before the full login handshake (and waiting for the handshake to avoid that error is bad for other reasons).