Add a new option that automatically adds a _created time stamp to each document when _rev is changed

[SinanGabel]

(1) What?

Every time a document has its _rev changed another key:value is updated, a _created time stamp with time in milliseconds since 1970.

{
_id: ...
_rev: ...
_created: 1598352308309
}

(2) Configuration option (choice) in local.ini

The configuration could e.g. be in the stats configuration part:
configuration stats

[stats]
doc_created = true
...

[wohali]

Duplicate of #1533