This unit is the interface for incoming and outgoing packets and manages the
DAQ.

There is one DAQ instance per active source (interface, pcap, etc.).  The
DAQ determines the required root decoder, instantiated upon thread
initialization, and which remains the same for all packets.

The other modules use the Active interface to detain packets. A packet will
not be held if it would drop the the available DAQ message pool down below 
the DAQ batch size. DAQ batch size (the number of packets Snort can process
in batch mode) can be configured using this command line option 
--daq-batch-size and the pool size is obtained using a DAQ API call: 
daq_instance_get_msg_pool_info(DAQ_Instance_h, DAQ_MsgPoolInfo_t)
