Just reading the code again in prep for some alloc reductions.
Change-Id: I065226ea794b7ec7144c2b15942d35131c9313a8
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Its semantics has changed slightly, this will let us use it to
drive batched logging in special circumstances.
Signed-off-by: David Crawshaw <crawshaw@tailscale.com>