udp_send: add a time window for errors

udp_send: add a time window for errors

This implements a grace period during which ICMP Destination Unreachable /
Port Unreachable are ignored from the peer, with the purpose of catching
 * synchronisation problems (e.g., receiver started after sender);
 * unforeseen events (e.g. delays, reboot, reconfiguration).

To avoid receiving persistent errors from unicast UDP clients, two time
windows are used:
 (a) The 'error-allowed' period t_A
     This time window starts when the first ECONNREFUSED error is seen
     and ends after t_A seconds. The value of t_A is a guess which should
     cover the expected time needed to sort any receiver problems out.
 (b) The 'error-free' period t_B
     During the t_B seconds following the interval t_A, no further
     connection errors are accepted; if an ECONNREFUSED is seen it
     will cause the target to be evicted from the list.

This windowing process restarts itself, i.e. the first error seen after
t_A+t_B will reset the counters.

The following examples illustrate the algorithm, where 'x' indicates
receipt of an ICMP error message.

 1) Some errors received during initial receiver setup

    |-x-x-x-x-x-x-x--|------------|------....
    |
    t_0              t_0 + t_A    t_0+t_A+t_B

    Since no errors are received after t_0 + t_A, no action will be taken.

 2) Persistent errors

    |-x-x-x-x-x-x-x-x|-x-x-x-x-x-x|
    |
    t_0              t_0 + t_A    t_0+t_A+t_B

    The first error received after t_0+t_A evicts the target.

 3) Recurring short errors

    |-x-x-x-x-x-x-x--|-//--|--------//---|-x-x-x-x-x-x-x--|------------|---...
    |
    t_0                    t_0+t_A+t_B   t_1              t_1+t_A      t_1+t_A+t_B

    Here the counter is reset at the first error after t_1. Since no more errors
    were seen after t_1+t_A, streaming continues.

For simplicity, the implementation uses t_A = t_B = 30 seconds.

The behaviour with an unavailable receiver is now:
May 24 18:08:16 (0) (2702) vss_send: sending 123:0 (548 bytes)
May 24 18:08:18 (0) (2702) vss_send: sending 132:2 (548 bytes)
May 24 18:08:18 (2) (2702) udp_check_socket_state: Evicting 10.0.0.2#8000 after 31 seconds of connection errors.

May 24 19:34:55 (0) (2702) vss_send: sending 5:14 (1232 bytes)
May 24 19:35:10 (0) (2702) vss_send: sending 11:3 (1232 bytes)
May 24 19:35:10 (2) (2702) udp_check_socket_state: Evicting 3ffe::2#8000 after 31 seconds of connection errors.