use exit==destination approach for mix for better anonymity

[chaitanyaprem]

Description

As per discussions here and in general for better anonymity, we should switch to configuration where the mix exit node is same as destination peer. This way the protocol handler i.e lightpush handler will be invoked from mix layer directly in same node.

Ref Milestone waku-org/pm#365, but since changes are available in mix we may as well take this change now instead of waiting to implement js-mix.

[github-actions]

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quay.io/wakuorg/nwaku-pr:3642

Built from 9297f30

[Ivansete-status]

LGTM! Thanks for it! 💯
Just added two nitpicks :D

[jm-clius]

This LGTM but is a design choice that has some impact on our assumptions (service nodes should support mix). Therefore I think the exit==destination choice should be described in the Waku-mix spec: https://github.com/waku-org/specs/blob/master/standards/core/mix.md

[chaitanyaprem]

This LGTM but is a design choice that has some impact on our assumptions (service nodes should support mix). Therefore I think the exit==destination choice should be described in the Waku-mix spec: https://github.com/waku-org/specs/blob/master/standards/core/mix.md

We have already indicated that relay nodes should mount mix, isn't that enough or should we be more explicit that service-nodes should mount mix as well?

[NagyZoltanPeter]

LGTM