This removes the feature where top nodes of the proof can be elided.
It was intended to be used by the LES server, to save bandwidth
when the client had already fetched parts of the state and only needed
some extra nodes to complete the proof. Alas, it never got implemented
in the client.
In this PR, all TryXXX(e.g. TryGet) APIs of trie are renamed to XXX(e.g. Get) with an error returned.
The original XXX(e.g. Get) APIs are renamed to MustXXX(e.g. MustGet) and does not return any error -- they print a log output. A future PR will change the behaviour to panic on errorrs.
