mem: allow using io.WriterTo with a io.LimitedReader

[GiedriusS]

The caller of this function can wrap the io.Reader in a io.LimitedReader. This happens if some max message size is set. If so, this io.WriterTo check doesn't work anymore. Work around this by checking if it is maybe a io.LimitedReader.

Overall, the problem I'm trying to solve is that the constant

		buf := pool.Get(readAllBufSize)

32KiB is way too much in our use case. Messages are typically at max about only 1KiB in size so we always overallocate by ~31KiB in the best case scenario so we want to use the io.WriterTo branch so that we could appropriately size the buffer.

Is this OK? Any suggestions on how to make the code prettier? Also, maybe some suggestions on how to do something like io.LimitedReader on the io.WriterTo?

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :warning: Please upload report for BASE (master@7472d57). Learn more about missing BASE report. :warning: Report is 15 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #8697   +/-   ##
=========================================
  Coverage          ?   83.25%           
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  Files             ?      416           
  Lines             ?    32306           
  Branches          ?        0           
=========================================
  Hits              ?    26895           
  Misses            ?     4027           
  Partials          ?     1384           

Files with missing lines	Coverage Δ
mem/buffer_slice.go	96.57% <100.00%> (ø)

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[GiedriusS]

@ash2k any thoughts?

[easwars]

@arjan-bal : Can you please take a first look and see if this seems ok.

[github-actions[bot]]

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