When to force out first counter sample

[sflow]

Currently we force out a counter sample on the first ‘tick’ in the main thread. Instead we might want to trigger that to happen only when the first VAPI call for that interface has succeeded (or has at least been attempted). This would prevent hsflowd from temporarily reporting a made up ifIndex number - which it will do for the samples between that first counter sample and the first one that has a Linux ifIndex annotation.