* Implement requests "create snapshot" and "discard snapshot" in tresor lib.
* Adapt tresor tester in order to test the new feature.
* Remove temporary code from tresor tester that skipped such requests with
the hint that they were not supported yet.
* Add mandatory "id" attribute to <request op="create_snapshot"/> and
<request op="discard_snapshot"/> tag. A "discard snapshot" command always
refers to the snapshot created by the "create snapshot" command with the
same "id" value.
* Clean-up command pool a bit.
Fix#4971
801a779a88