obscache: Only access the new obsmarkers for marker update Since we know what what is the part of the file with "new" markers we can just access these. If the cache was up to date before the transaction, we'll re-parse data we just wrote. See the inline comment for details. This is good enough for now.

obscache: extract _updatemarkers code into its own function This split the update logic from the bit retrieving markers. That cannot be bad.

obscache: extract code to update from new revision Code cleanup and prepare upstreaming. The make the code more likely to work with the key validation returning an iterator of revs to update the cache with.