The `ioutil.NopCloser(reader)` was hiding nested hash readers.
We make it an `io.Closer` so it can be attached without wrapping
and allows for nesting, by merging the requests.
Verify() was being called by caller after the data
has been successfully read after io.EOF. This disconnection
opens a race under concurrent access to such an object.
Verification is not necessary outside of Read() call,
we can simply just do checksum verification right inside
Read() call at io.EOF.
This approach simplifies the usage.
