Make delete idempotent

[Stebalien]

Currently, delete on a non-existent key returns ErrNotFound. However, this isn't really all that useful. Additionally,

1. badgerdb doesn't return an error when deleting a non-existent and making it do so will be a slight performance hit.
2. Feat: Implement a PersistentDatastore by adding DiskUsage method #27 will cause deletes to succeed, sometimes, even when there was no value to be deleted (e.g., a concurrent delete and put where we resolve the put after the delete).

[whyrusleeping]

hrm... I was against this until you threw my weakness at me. I love making things idempotent...

[kevina]

I think we should fix what every in #27 that causes delete to not return the correct value. I am against returning less information than we know.

At the specification level I would say that a datastore should have an option of not returning ErrNotFound but it should be allowed to. If it does return ErrNotFound it should do so consistently.

[Stebalien]

Moving to: ipfs/go-datastore#104