Fix barrier tracking for flow deletion

Reviewer: @kenchiang

Flow deletion was previously ignored by barriers. I noticed this when implementing delayed kflow revalidation in IVS, which does flow add/modify/delete callbacks asynchronously.

It's unfortunate I had to dup the flow_delete for every flow. We don't currently have an object that's a singleton per request - the ptr_cxn contains the flow entry so many of them are created for each request, but it also owns a LOCI request object. The fix would be to decouple the per-callback state from the per-request state. That's a big change I didn't want to add to this pull request.