sync: Add ServiceGroup implementation

[sebastianst]

We're often in a situation where we want to start several services in goroutines and then wait for either all of them to return with a nil error or the first to return a non-nil error. So far, we usually either use a bare error channel in such situations or the errors.Gatherer together with WaitDoneOrFail. However, the Gatherer has a rather bloated implementation with several mutexes and other synchronization state and its multi-error feature doesn't help in such situations because there will only be one error once the first routine fails.

So I propose to add a lightweight implementation ServiceGroup that covers this exact use case. It is an eagerly-finishing version of the golang.org/x/sync/errgroup.Group, which always waits for all routines to return.

Don't merge yet, untested and intended as a base for discussion.