Flag packages as abandoned in lookup results

[rarkins]

Describe the proposed change(s).

For each package we look up, we should calculate the most recent release date of all releases in the result (where releaseTimestamp is present). This should not be "the release date of the release flagged as latest" - instead it should be the latest/most recent release timestamp of all releases in the package, including beta/non-latest releases. This field could be stored as lastReleaseTimestamp at the package level. It should be calculated and stored before filtering such as allowedVersions is applied. i.e. in the datasource/index processing before passing back to the worker lookup process.

In addition, a new configuration option should be added: abandonmentThreshold, which supports the same date range syntax as minimumReleaseAge. It should default to null (no threshold) but if it is defined then it should be used to calculate a new isAbandoned boolean field to be added to packages, based on the lastReleaseTimestamp value calculated earlier. The field isAbandoned should be present (true or false) only if both lastReleaseTimestamp and abandonmentThreshold are defined for a package - otherwise undefined. Such logic could be done in the worker and is not essential in the datasource itself.