Enable full IPv6 support for synapse

[matrixbot]

This issue has been migrated from #13107.

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Description:

At the moment synapse cannot be used with workers in IPv6 only environments, as the connection to redis fails and thus no communication is possible. There are various issues existing in the synapse repository and some in the twisted repository.

Feature

• Allow fully functional, worker based setup of synapse in IPv6 only environments
• Ensure that the functionality is covered by unit/integration tests

Bug Bounty

I would like to put a bounty on solving this bug to finally enable us and the community to run synapse properly in IPv6 only environments. I have an amount in CHF in my mind which I think is appropriate, but if you are interested in solving this as a paid gig, please reach out to me at ipv6only-bounty at ungleich.ch with a proposal on the time frame until it can be solved and which amount you think is appropriate.