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.
Golang 1.6 is default version for the build now.
Additionally set 'GODEBUG=cgocheck=0' for now, until
we fix the erasure coding package.
Readmore here https://tip.golang.org/doc/go1.6#cgo
Harshavardhana