Limit spamming GUIs with boring updates to once in 3 seconds, unless
the notification is relatively interesting and the GUI should update
immediately.
This is basically @barnstar's #14119 but with the logic moved to be
per-watch-session (since the bit is per session), rather than
globally. And this distinguishes notable Notify messages (such as
state changes) and makes them send immediately.
Updates tailscale/corp#24553
Change-Id: I79cac52cce85280ce351e65e76ea11e107b00b49
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
