Add support for <cancel-commit> operation (v1.1)

The <cancel-commit> operation is defined as a part of :confirmed-commit:1.1 in RFC 6241:

Cancels an ongoing confirmed commit.  If the <persist-id>
         parameter is not given, the <cancel-commit> operation MUST be
         issued on the same session that issued the confirmed commit.

It can be used to explicitly the configuration to its state before the confirmed commit
without waiting for the confirm timeout to expire.