TitleSpeedup SNMP scan by bulk fetching of OIDs

The SNMP scan needs currently a lot of round trips (> 10) because some of the checks need to fetch non-standard OIDs. We should speed up this. Only idea is to fetch multiple OIDs in parallel. SNMP GET can do this. In SNMP v2c the response packet can also contain multiple error messages of type "NoSuchOID". v1 only allows *one* error message per packet. So that legacy devices need one round trip per non-existing OID. But since the usual case is v2c this optimization should work. Implemention: Check fetching non-standard-OIDs during scan need to declare them in check_info. Check_MK prefetches all these and also the standard OIDs into the OID cache.