The stats reporter only installed the connstats callback on the TUN
device after the report loop negotiated an interval with the server.
Traffic that flowed before that point (e.g. an SSH handshake) was
silently dropped because the TUN wrapper's stats.Load() returned nil.
We now install the connstats callback immediately and we no longer
re-install the callback every interval unless the interval changed.
Fixes flaky TestAgent_Stats_SSH, TestAgent_Stats_ReconnectingPTY,
and TestAgent_Stats_Magic by ensuring the connstats callback is
always installed before network traffic can flow.
Closescoder/internal#505
Closes CODAGT-517
Mathias Fredriksson
20d678b886