Because wgLock is held while some wireguard-go methods run,
trying to hold wgLock during HandshakeDone potentially creates
lock cycles between wgengine and internals of wireguard-go.
Arguably wireguard-go should call HandshakeDone in a new goroutine,
but until its API promises that, don't make any assumptions here.
Maybe for #110.
Signed-off-by: David Crawshaw <crawshaw@tailscale.com>