This starts adding flow tracking stats, without exposing them anywhere
yet. Flow structs are created as needed and metrics are bumped, and
benchmarks show no change in performance.
Updates #3560
Change-Id: I376187a8452ec92d49effcbf48a6fb4f4d787b8a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
