scc check-prs caching

This PR should implement some minimal caching mechanism for `scc check-prs`:

- creates new attribute`rebasedprs`, i.e. a set of all merged PRs rebased to another merged PR with matching links
- stores all the `links` to the `rebasedprs` set to a cache file using `pickle.dump`
- reloads the stored links from the cache file and updates `self.links` and `self.rebasedprs`
- cleans deprecated code using `git notes` in favor of this mechanism

Notes about PR caching:
- all cached rebased PRs should be skipped from the rebased check which should save time and API calls
- all unrebased PRs, rebased but opened PRs and  PRs with mismatching comments will still be rechecked 
- the cache should hopefully be updated every time the job is run

To test this PR:
```
scc check-prs dev_5_0 develop --cache-dir /tmp
```