In IOC::write/append, add ability to merge on the fly

If a journal entry is large enough (TBD what that means), it would save a good deal of IO to have write & append merge it then rather than wait for it to be merged by Synchronizer.

Ex: an object that is 5MB, where a write() op would replace all 5MB of it. Obviously it would be more efficient to replace the whole object then with data already in mem rather than write the journal entry, then make Sync read the obj and the journal later.