Use thread cap only for deault value

Currently thread cap of 16 is applied to record.processing.threads, unless AVAILABLE_CORES is provided. Allow the user to configure whatever number of processing threads he wants. It should work as follows:

• if the record.processing.threads is specified, the value provided by the user is used
• if the record.processing.threads option is not specified, the default value of Math.min(16, AVAILABLE_CORES) is used.