This PR introduces a node scheme abstraction. The interface is only implemented by `hashScheme` at the moment, but will be extended by `pathScheme` very soon.

Apart from that, a few changes are also included which is worth mentioning:

-  port the changes in the stacktrie, tracking the path prefix of nodes during commit
-  use ethdb.Database for constructing trie.Database. This is not necessary right now, but it is required for path-based used to open reverse diff freezer