Add transaction support

[Shelnutt2]

Add transaction support. Items should be built and stored until commit, then added to the map. We also should only flush the map on commit.

[Shelnutt2]

Since a key can not be removed from the key-value map, transactions are going to end up ugly.

The basic idea so far is:

1. Use a transaction_dir
2. Create a temp map inside dir with uuid+timestamp
3. Add rows to temp map
4. On commit, we will then loop through temp map and add to new map
5. Finally we call flush() to push to disk of final map

The problems are for group commits, we if need to rollback after one table has already committed we must now go back and set the delete markers. This means rollbacks will touch every single row, and they are not atomic either.