In IOC::read(), merge objects on the fly

IOC::read() involves loading a portion of an object and merging it with any existing journal data. There are cases where it makes sense to write the merged data as part of the read() rather than make Sync do it.

Ex: read() would load a whole object with a large journal. ~90% of the work that Sync does is done by read() in that case; read() should do the remaining 10% and not make Sync redo that 90%.