NIFI-3545: Release M FlowFilews once N signals arrive

[ijokarumawak]

• Support multiplle incoming FlowFiles to Wait processor, up to Wait
  Buffer Count
• Added Releasable FlowFile Count, which controls how many FlowFiles can
  be released when wait condition is met
• Added special meaning to Notify delta Zero(0) to clear a signal
  counter back to zero

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[ijokarumawak]

Dear reviewer,

I've tested this PR with different use-cases. Templates and screenshots are available at this Gist.

Thanks in advance!

[apiri]

reviewing

[apiri]

Was able to test this in a variety of situations and using your supplied templates. Works as anticipated and thanks for all the tests. Will get this merged in! Thanks!