Possible use cases:
* Matching comments between PHP and Parsoid HTML [implemented here]
* Finding the same comment in a different revision of a page
(e.g. while resolving an edit conflict, or to allow resuming
composition of autosaved comments) [implemented for highlighting
user's own posted comment only]
* Permanent links to comments [future]
The reasoning for this form of ID is:
* _Timestamp_ by itself is a nearly unique identifier, so it's a good
thing to start with
* Users may post multiple comments in one edit (or in many edits in
one minute), so we need the _sequential number_ to distinguish them
* _Username_ is probably not required, but it may reduce the need
for sequential numbers, and will help with human-readability if we
add permanent links
The ID remains stable when a new comment is added anywhere by anyone
(excepts comments within the same minute by the same user), or when a
section is renamed.
It's not always stable when a comment is moved or when an entire
section is moved or deleted (archived), but you can't have everything.
Change-Id: Idaae6427d659d12b82e37f1791bd03833632c7c0
