Change brings in a new signVerifyReader which provides a io.Reader
compatible reader, additionally implements Verify() function.
Verify() function validates the signature present in the incoming
request. This approach is choosen to avoid complexities involved
in using io.Pipe().
Thanks to Krishna for his inputs on this.
Fixes#2058Fixes#2054Fixes#2087
Signature calculation has now moved out from being a package to
top-level as a layered mechanism.
In case of payload calculation with body, go-routines are initiated
to simultaneously write and calculate shasum. Errors are sent
over the writer so that the lower layer removes the temporary files
properly.
Harshavardhana