[SDKs] CollectionFS should repack highly fragmented files
Currently, a sequence of alternating "write" and "snapshot" operations will result in a very fragmented collection (i.e., lots of small blocks).
#10181 is one real-world example of this pattern.
CollectionFS should provide an API for repacking fragmented file data. This should also be invoked automatically in certain circumstances (e.g., when a file is already highly fragmented during a sync/snapshot).