Add constructors with maxTime

Motivation:
Improve the default constructor behavior, proposing new constructor to
set the maxTime as creation time.
Reusing MINIMAL_WAIT also in TrafficCounter.

Modifications:
Since other properties (writeLimit, readLimit, checkInterval) are not
volatile, I propose to let maxTime non volatile too.
Replace "10ms" fixed value by
AbstractTrafficShapingHandler.MINIMAL_WAIT.

Result:
Ability to specify maxTime from constructor step, or just after using
setMaxTimeWait