phases: add a 'registernew' method to set new phases This new function will be used by code that adds new changesets. It ajusts the phase boundary to make sure added changesets are at least in their target phase (they end up in an higher phase if their parents are in a higher phase). Having a dedicated function also simplify the phases tracking. All the new nodes are passed as argument, so we know that all of them needs to have their new phase registered. We also know that no other nodes will be affected, so no extra computation are needed. This function differ from 'retractboundary' where some nodes might change phase while some other might not. It can also affect nodes not passed as parameters. These simplification also apply to the computation itself. For now we use '_retractboundary' there by convenience, but we may introduces simpler code later. While registering new revisions, we still need to check the actual phases of the added node because it might be higher than the target phase (eg: target is draft but parent is secret). We will migrate users over the next changesets.