Cleanup remaining issues in StreamPipeReader/StreamPipeWriter

[jkotalik]

Epic #8829

There are a few more issues that need to be addressed:

• Add MaxBufferSize setting
• OnReaderCompleted
• OnWriterCompleted
• Throw on overlapping Reads/Flush
• Refactor ReadAsync to avoid an extra async state machine.
• Creating a new tail segment if we have consumed a certain percentage of the segment.
• Remove _completedSegments.RemoveAt(0); from StreamPipeWriter

[analogrelay]

What's the story here? @jkotalik @davidfowl