Add ensureSingleBuffer in slave class

[slacrherbst]

The ensureSingleBuffer call exists in the stream Master. But the documentation infers, correctly, that it can and should be used in the Slave as well. This adds a slave specific version in the Slave class.

This also adds a more meaningful call to allocate a frame inside a Slave from the local Slave pool: reqLocalFrame()

[codecov-commenter]

Codecov Report

Merging #965 (f7ee6e9) into pre-release (761c5c2) will not change coverage.
Report is 4 commits behind head on pre-release.
The diff coverage is n/a.

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@@             Coverage Diff              @@
##           pre-release     #965   +/-   ##
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  Coverage        29.89%   29.89%           
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  Files               65       65           
  Lines             6624     6624           
  Branches          1013     1013           
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  Hits              1980     1980           
  Misses            4488     4488           
  Partials           156      156