Maintain closed state and add isClosed property

[gmaclennan]

After the indexer is closed, it won't work any more. However currently behaviour is undefined because we don't keep a reference of the closed state. This is a non-urgent issue, because it's not causing any problems, this is just a question of completeness. Methods should throw an error once it is closed.

[gmaclennan]

Revisiting this, since the unlink() method requires the indexer to be closed, we should properly track the closing and closed states, and throw for other methods where relevant if state === 'closing' || state === 'closed' and throw for unlink() if state !== 'closed'. @EvanHahn

[EvanHahn]

Addressed in #42.