Fix NMS-6194: The threshold processor doesn't work well with complex JEXL Expressions

Bonus Features:

- Added an optional attribute called 'relaxed' (default: false), to tell
  the threshold processor to force the evaluation of the formula even if
  some data sources are missing (which can be useful for conditional
  complex formulas).
- Decreased log level for some anoying warning related with threshold
  processing.
- Improved some INFO/DEBUG messages related with thresholding.
- Make the custom resource filter parameters (like id, ipaddr,
  nodeid, iflabel) to match their name ignoring case.
- Be able to use snmpifspeed and snmpiftype from the snmpinterface table
  on threshold formulas for SNMP Interface resources.
- Updated Apache JEXL to 2.1.1 (from 2.0.1)