read_stream: Move logic about IO combining & issuing to helpers

read_stream: Move logic about IO combining & issuing to helpers

The long if statements were hard to read and hard to document. Splitting them
into inline helpers makes it much easier to explain each part separately.

This is done in preparation for making the logic more complicated...

Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/f3xxfrkafjxpyqxywcxricxgyizjirfceychyxsgn7bwjp5eda@kwbduhy7tfmu