Persisted confirmed commit support (v1.1)

The RFC-6241 defines the feature as:

If the <persist> element is given in the confirmed <commit> operation, a
follow-up commit and the confirming commit can be given on any
session, and they MUST include a <persist-id> element with a value
equal to the given value of the <persist> element.

This requires to save previous DS state and timeout along with persistID.

Unlike in case of ordinary confirmed commit, the server must not restore the configuration

if session is terminated before confirm commit timeout:

If the session issuing the confirmed commit is terminated for any
reason before the confirm timeout expires, the server MUST restore
the configuration to its state before the confirmed commit was
issued, *unless the confirmed commit also included a <persist>
element*.

Support for persisted commit implies that user might be able to restore rollback any of them using

<cancel-commit> operation.