This also updates all usages of the deprecated Val ctor to use
either IntervalVal, TimeVal, or DoubleVal ctors. The reason for
doing away with the old constructor is that using it with TYPE_INTERVAL
isn't strictly correct since there exists a more specific subclass,
IntervalVal, with overriden ValDescribe() method that ought to be used
to print such values in a more descriptive way.
Connection durations were being compared "less than" instead of "greater
than" the desired duration threshold as provided to
set_current_conn_bytes_threshold() or
ConnThreshold::set_duration_threshold()
Now, in addition to setting thresholds for bytes and packet, one can set
a threshold for connection duration. Note that the threshold event is
only raised once the next packet in the connection is seen.
This also fixes a small pre-existing bug, in which a bunch of warnings
were raised if someone just used the lower-level functions without going
through the higher-level scripting API.
