Proposed fix for JENKINS-24401

The Depot class now has a method for obtaining the server's timezone GMT
offset. This offset is then applied to the changelist's submit time. Thanks to
that, the submit time of a changelist in the build's changelog appears in the
Jenkins server's timezone.
It makes sense to move the in-code comment to Javadoc.
I'd also recommend to add <code>@since</code> to new methods