Close() does not return an error, probably indicative of latent bugs

It appears Close() doesn't return an error, but it also appears this is from gocbcore's Agent that doesn't return an error.  Closing something might fail which wouldn't be communicated upstream. 

Also, note that it could be an io.Closer.

Recommend running `go vet` and fixing those, followed by `golint` to make things more consistent.  

Could also run github.com/kisielk/errcheck which shows the places errors are dropped.