It was a really a mutable field owned by mapSession that we didn't move
in earlier commits.
Once moved, it's then possible to de-func-ify the code and turn it into
a regular method rather than an installed optional hook.
Noticed while working to move map session lifetimes out of
Direct.sendMapRequest's single-HTTP-connection scope.
Updates #7175
Updates #cleanup
Change-Id: I6446b15793953d88d1cabf94b5943bb3ccac3ad9
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
c87d58063a