make subthreshold annotations independent; fixes #263

@leo-singer this is what I have from our discussion last Friday about making the subthreshold annotations independent.

I have restricted myself only to the annotation part. The inclusion of sending internal alert is an additional layer. Maybe !669 (closed) could be added on top of this.

Let me know what you think.

Also pinging reviewers and chairs @erik-katsavounidis @shaon.ghosh @roberto.depietri @stuart.anderson The description of the idea is there in #263 (closed)