various checks can hang server pollers and agents

snmp checks, some agent checks (system.run on windows) and possibly other types can hang server pollers and agents for prolonged periods of time.

when fixing, this issue will have to be split up for these known issues :

1. snmp checks hanging;
2. user parameters hanging an agent;
3. system.run not properly timing out on windows

this was fixed in ZBX-3446 for external checks, this is a followup issue for other types