Require opt-in for comment flood tracking

[fjarrett]

Up to this point, Stream has been creating a record each time Comment flooding is prevented. That is to say, when WordPress prevents a user from posting duplicate comments.

When WordPress prevents comment flooding, it's preventing the comment from entering the database.

Although this is somewhat nice information to know, tracking it actually falls outside the scope of Stream's purpose, which is to only track changes to the database (see #547 for a related discussion on this topic).

Tracking comment flooding actually isn't a smart idea for the majority of sites because it ends up creating a flood of Stream records which then drown out the important records dealing with actual database changes.

On my personal site, comment flood records accounted for 36% of all records. Knowing that WordPress prevented flooding is not worth 36% of real estate in my Stream data.

The point of preventing comment flooding is to minimize noise in the WP Admin area by weeding out superfluous comments. Stream should not be adding noise by tracking this behavior by default.

This callback should be opt-in only, similarly to what was done in #649.