refactor(7181): move streaming_merge() into separate mod from the merge node #7799
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Which issue does this PR close?
Prerequisite for the cascading merge.
Part of #7181 .
Rationale for this change
Create a new merge submodule which has the SortPreservingMergeStream (a.k.a. the loser tree). The SortPreservingMergeStream will be used repeatedly as part of the cascading merge tree.
By comparison, the streaming_merge() method will only be called once -- by the cascading struct (in the cascade submodule).
What changes are included in this PR?
See above.
Are these changes tested?
Only moving code. Nothing change.
Are there any user-facing changes?
No.