sd-varlink: async upgrade support (#42974)

With 2c6f9af8e5425c2086fbc8ca496843f162e4af9b sd-varlink gained protocol
upgrade support, however only in a synchronous fashion. This adds
asynchronous protocol upgrade for the server side, thus enabling
multiplexing daemons (i.e. those that handle multiple connections from
the same event loop) to support protocol upgrades too.

I plan to use this in the upcoming "ptybroker" component that allows
acquiring a pty through varlink.

/cc @mvo5