connectx4.lua: add statistics counters
connectx4.lua: add statistics counters

Add the same set of counters supported by the Intel driver.  Add a
pull() method to the ConnextX4 object to periodically sync the stats
so the "control" app can be run as a regular Snabb app.

SnabbCo/snabbswitch,eugeneia/snabb,eugeneia/snabb,Igalia/snabb,Igalia/snabb,eugeneia/snabb,eugeneia/snabb,eugeneia/snabb,snabbco/snabb,snabbco/snabb,eugeneia/snabb,snabbco/snabb,snabbco/snabb,Igalia/snabb,Igalia/snabb,snabbco/snabb,Igalia/snabb,eugeneia/snabb,SnabbCo/snabbswitch,eugeneia/snabb,SnabbCo/snabbswitch,Igalia/snabb,SnabbCo/snabbswitch,Igalia/snabb,snabbco/snabb,snabbco/snabb,Igalia/snabb,snabbco/snabb