worker: implement _blockingreader.readinto() (issue6444)

The core logic for readinto() was already implemented in read(), so this is
mostly extracting that code into its own method.

Another fix for issue6444 was committed to the stable branch: 2fe4efaa59af.
That is a minimal fix that implements readinto() only on Python versions that
require readinto() (3.8.0 and 3.8.1), which is the right approach for the
stable branch. However, I think that this changeset has its value. It improves
performance in cases when pickle can use readinto(), it reduces code
duplication compared to the other patch, and by defining readinto() on all
Python versions, it makes behavior more consistent across all Python versions.
This changesets reverts the other change.
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