activity: self activity should change recency

[arthyn]

Fixes TLON-2088 by changing the way we give recency values in that if the "read" floor is greater than the last message, make that the recency. However to get our %read action to put a value other than the last message time, I had to change the action type slightly.

PR Checklist

• Includes changes to desk files
• Describes how you tested the PR locally (test ship vs livenet)
• If a new feature, includes automated tests
• Comments added anywhere logic may be confusing without context

[linear]

TLON-2088 activity: recency not reflected through actions from ourselves

[Fang-]

Hoon-side changes here look good, thanks for the fix!