In that commit, AtomicPutCreate takes previous = nil to Atomically create keys
that don't exist. We need a create operation that is atomic to prevent races
between multiple libnetworks creating the same object.
Previously, we just created new KVs with an index of 0 and wrote them to the
datastore. Consul accepts this behaviour and interprets index of 0 as
non-existing, but other data backends do no.
- Add Exists() to the KV interface. SetIndex() should also modify a KV so
that it exists.
- Call SetIndex() from within the GetObject() method on DataStore interface.
- This ensures objects have the updated values for exists and index.
- Add SetValue() to the KV interface. This allows implementers to define
their own method to marshall and unmarshall (as bitseq and allocator have).
- Update existing users of the DataStore (endpoint, network, bitseq,
allocator, ov_network) to new interfaces.
- Fix UTs.
