Currently, the worker protocol has a version number that we increment
whenever we change something in the protocol. However, this can cause
a collision between Nix PRs / forks that make protocol changes
(e.g. PR #9857 increments the version, which could collide with
another PR). So instead, the client and daemon now exchange a set of
protocol features (such as `auth-forwarding`). They will use the
intersection of the sets of features, i.e. the features they both
support.
Note that protocol features are completely distinct from
`ExperimentalFeature`s.
This increases test coverage, and gets the worker protocol ready to be
used by Hydra.
Why don't we just try to use the store interface in Hydra? Well, the
problem is that the store interface works on connection pools, with each
opreation getting potentially a different connection, but the way temp
roots work requires that we keep one logical "transaction" (temp root
session) using the same connection.
The longer-term solution probably is making connections themselves
implement the store interface, but that is something that builds on
this, so I feel OK that this is not churn in the wrong direction.
Fixes#9584
