Fix compaction throttle

Compaction throttling is broken in the following ways:

• It throttles bytes read after being decompressed
• Compaction creates multiple scanners which share the rate limiter causing too much throttling
• It bears no resemblance to the reported compaction time remaining calculation (Bytes of source sstables processed since start of compaction)

To fix this we need to simplify the throttling to be only at the CompactionIterator level.