Redesign the patch life cycle

- there are too many states, which are sometimes not orthogonal (a patch can be both RFC and under review). This needs to be simplified, eg. new, under review, reviewed, needs work (aka changes requested), merged (aka accepted), rejected, superseded.
- State change needs to happen automatically when possible. eg.
- reviewer assigned -> change the state to under review
- r-b tag -> change the state to Reviewed
- new revision of a series -> change states of previous versions to superseded
- patch in tree -> merged (still problems to solve there)
- log state changes in the EventLog API