Break up SeekableStreamIndexTaskRunner.runInternal()

This method has 550+ lines of code. Its cyclomatic complexity is 74 (the reasonable limit of cyclomatic complexity for methods is considered 14).

@jihoonson @gianm probably you are the only persons who understand this method. It would probably take days for anybody else to understand it to the level to be able to make any change in it with confidence.

Start by locating SeekableStreamIndexTaskRunner.runInternal(), the entry point named in the issue, and read the surrounding class to understand its responsibilities. The issue provides no file path or tests; completion would require breaking up the 550+ line method while preserving its behavior and reducing its cyclomatic complexity.

Written by the indexing model from the issue text.