Add internal API for note iteration

Add internal API for note iteration

Some algorithms need to be able to iterate through all notes within a
particular bucket, such as when splitting or combining a bucket.
Exposing an Iterator<Note> makes this traversal possible.

For a LeafBucket the iteration is simple, its over the sorted array of
elements.  For FanoutBucket its a bit more complex as the iteration
needs to union the iterators of each fanout bucket, lazily loading any
buckets that aren't already in-memory.

Change-Id: I3d5279b11984f44dcf0ddb14a82a4b4e51d4632d
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>