S3 readme

Don't merge. Just for @joshblum to look at. Following are some comments copied over from Phab:
- store if delete oper
- store if delete operation is contained in sst file
- ssfile key range is max of delete operation or latest event
- store history in metadata (the files that were read and written)
- single compactor for all nodes
- some way to figure out if there exists a delete operation in some key range
- parallel compaction for each stream, level, non-overlapping key ranges
- sqs stuff -- figure it out
- fix the file data format description, data file only contains compressed chunks, index file is an index of those chunks
- also store if that chunk contains a delete operation