Remove clockwork dependency

[vppawar]

In this PR,

• Remove the dependency on “github.com/jonboulle/clockwork - a fake clock for golang” The clockwork allows to fake the real clock and it is used for testing the digital signature.

• Add the changelog

Reason to remove the clockwork dep The goxmldsig library is used in our product , so this fake clock gets directly plugged into our production code. This creates the backdoor and we do not want it.

[russellhaering]

Closing this out, I'm not clear how having a mockable clock creates a "backdoor".