Support compaction callback that allows a client to modify a doc before moving it into a new file

The current callback implementation supports an option of calling a callback function right after writing each doc to the new file. This means that it doesn’t allow an application to modify a doc before being copied to the new file. However, as both Couchbase server and mobile also need to modify a doc (e.g., logically deleting an expired item) before moving a doc to the new file, we need to support an additional option of calling a callback function before copying a doc.