Add IPv6 support to StatusServer and related classes.

[moodyjon]

Fixes #117
Update: Fixes #121

Not sure whether this is complete. I think I need help with this part in hub/env.py:

        if result == 'localhost':
            # 'localhost' resolves to ::1 (ipv6) on many systems, which fails on default setup of
            # docker, using 127.0.0.1 instead forces ipv4
            result = '127.0.0.1'

If I allow the cs_host() to be localhost instead of 127.0.0.1, then it will start TCP servers on both v4/v6 sockets.

What is the problem with docker and this behavior?

[moodyjon]

Just saw a new EADDRINUSE problem when running lbry-sdk tests against this hub branch.

https://github.com/moodyjon/lbry-sdk/actions/runs/3911989925/jobs/6686050669

Converting this to draft status while investigating.

[moodyjon]

I did some work on StatusServer.start() to make it retry to overcome transient EADDRINUSE. It got a bit more complicated in order to handle cases like HOST=::,0.0.0.0, HOST=0.0.0.0,::, HOST=myserver1,myserver2, etc. I imported resolve_host() from my moodyjon/lbry-sdk:ipv6 branch to do name resolution.

[moodyjon]

Have not seen EADDRINUSE since that one occurrence. Not sure what the root cause of that was. Possible infrastructure?

[moodyjon]

If I don't disable IPv4 mapping, then the IPv6 port will sometimes seize (or try to seize) responsibility for IPv4. The bad effect is seen when starting the server with HOST=::,0.0.0.0 or HOST=0.0.0.0,::. I forget which one fails, but 1 of these two orderings has persistent EADDRINUSE errors.

[moodyjon]

The bad ordering (on my machine) is HOST=::,0.0.0.0:

2023-01-17 10:14:49,962 - hub.herald.udp - WARNING - started udp6 status server on ('::', 50002)
2023-01-17 10:14:49,963 - hub.herald.udp - ERROR - UDP status server failed to listen on (0.0.0.0:50002) : [Errno 48] Address already in use
2023-01-17 10:14:52,965 - hub.herald.udp - WARNING - started udp6 status server on ('::', 50002)
2023-01-17 10:14:52,966 - hub.herald.udp - ERROR - UDP status server failed to listen on (0.0.0.0:50002) : [Errno 48] Address already in use

HOST=0.0.0.0,:: works fine.