Fix NoteBuilder leaks and callback cleanup

[alltheseas]

 Changes ensure the drop callback matches the actual FnMut(u64) signature and
  stop leaking NoteBuilder buffers by freeing them on failure/drop and clearing
  ownership once the note takes over (src/ndb.rs:39, src/note.rs:371, src/
  note.rs:484, src/note.rs:540, src/note.rs:543). Tests run: cargo fmt; cargo
  test.

should address #49