Major Compaction checkpoints for restartable compactions

[dlmarion]

For a major compaction on a tablet, if we captured:

1. the set of input files
2. the output file
3. the configuration (iterators, etc)
4. the read offsets in the input files (updated periodically)

Then we might be able to re-start a major compaction that failed in certain conditions (tserver shutdown, tablet re-hosting, etc.)