Pipelets: no_add usage, fix add_source() for assynchronous fetching, details:
  - optimize stateless pipelets by not requesting adding their source, setting option no_add to true
  - stateful pipelets now need to set no_add to false to force data fetching from their source
    during add_source()
  - add_source():
    - call _add_source() immediatly so that, if a downsteam pipelet trys to fetch() before
      this pipelet has finished fetching, that a new fetch be requested to this source.
    - improve documentation, in particular regarding option no_add that is now used also
      by stateless pipelets to prevent unnecessary fetching of their source
    - refactor
  - build(): call add_source() with option no_add if the newly created pipelet has set own property
    no_add.
  - trace(): add a constructor trace
  - delay(): add a constructor debug trace