remove ThreadSafeDatastore

[Stebalien]

It's a lie! We:

1. Assume that our datastores are thread-safe all over the place, not bothering to check for this interface.
2. Implement this interface for, e.g., the mount datastore that may not be thread-safe (depending on the sub-datastores).

Basically, there's no sane way to to do something like this in go. What we want is:

pub trait ThreadSafe {}

struct MyWrapper<D: Datastore> { ... }

impl<D: Datastore> ThreadSafe for MyWrapper<D> where D: ThreadSafe {}

Actually, we don't even need this because rust has already done all the hard
work with the Sync trait.

....

But we're using go which barely has types.

---

For completeness, it's actually possible to do this in go:

type threadSafeMixin struct{}
func (threadSafeMixin) ThreadSafe() {}

func NewWrapper(d Datastore) Datastore {
  if _, ok := d.(ThreadSafe) {
    return &struct{myWrapper, threadSafeMixin}{myWrapper{d}, threadSafeMixin{}}
  }
  return &myWrapper{d}
}

Let's not.

---

Merge first:

• remove thread-safe assertion go-ds-flatfs#53
• remove thread-safe assertion go-ds-redis#4
• remove thread-safe assertion go-ds-badger#55
• remove thread-safe assertion go-ds-leveldb#27

[hsanjuan]

Heads up that this was a breaking API change and was released under a patch version bump (consider doing a minor release bump in that case, as most of us don't expect builds to break after go get -u=patch.

[Stebalien]

You're right, I should have done that. I'm too used to versions meaning nothing at this point...