NpgsqlConnection.ConnectionString parsing fails for IPv6 hosts

[Brar]

This was broken by changes introduced with the multiple hosts feature.

[roji]

The ConnectionString_Host tests are failing on my local machine (and the test case for 127.0.0.1,127.0.0.1:5432,::1,[::1],[::1]:5432,localhost,localhost:5432 even hangs...).

@Brar @vonzshik are you not seeing this on your machines? Interesting that this isn't happening on CI either...

[Brar]

No, it worked after the fixes from @vonzshik

[vonzshik]

@roji everything's green on my end...

[roji]

I've temporarily disabled the test in #3824, will revisit this later to investigate what's special about my setup...