hub-generated release note from an incremental release is not complete

[erikyao]

Related Issue: #103

Fix to #103 is to count the documents in post-index steps and write the stats into ES mapping metadata as well as into the build docs in MongoDB.

However that fix works only for full releases where ES indexing take places (and thus a post-index operation will be performed). In an incremental release, diff results (i.e. patches) are generated from two MongoDB collections, but we cannot calculate the ES stats from the diffs (e.g. an updated doc in MongoDB might be filtered out while indexing).

A possible workaround is to calculate the ES stats after the diffs are applied to the involved indices.