Fixed ack for tombstones in cython-stream

[trauter]

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Description

Previously, Tombstones have neither been forwarded to the processor nor ack'ed.
copy of robinhood/faust#547

NOTE: All patches should be made against master, not a maintenance branch like
3.1, 2.5, etc. That is unless the bug is already fixed in master, but not in
that version series.

If it fixes a bug or resolves a feature request,
be sure to link to that issue via (Fixes #4412) for example.

[codecov-io]

Codecov Report

Merging #39 (607be47) into master (18230a7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #39   +/-   ##
=======================================
  Coverage   63.94%   63.94%           
=======================================
  Files          99       99           
  Lines       10274    10274           
  Branches     1154     1154           
=======================================
  Hits         6570     6570           
  Misses       3549     3549           
  Partials      155      155           

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