Cleanup, fixes and new features around Sinks

In details :

• small fixes on the BaseSink interface
• new regular flushing feature for BaseSink
  • a new flush method was added, called both at the end (finalize) and regularly if auto flush is activated (via non 0 AutoFlushPeriod property)
• new unit testing for BaseSink, both for the regular flushing and previous existing (and not yet tested) features
• replaced virtual methods toJSON, reset and mergeAndReset with free functions
• adapted existing Sinks to new features