MemoryDB can't derive default

[dvdplm]

Due to how we handle null nodes in memorydb – by calculating them on instantiation – we can't use MemoryDB::default() as a synonym for MemoryDB::new().

Do one of three things:

1. fix the null node instantiation hack (come talk to me or @rphmeier first)
2. remove #[derive(Default)] and fix all code that uses MemoryDB::default()
3. implement Default properly