Refactoring: General IO is now split from Networking IO

Make clear when the panic occurs to a missing trie root.

Add flush function.

Fix for assumption that empty trie root RLP can always be looked up.

updated to rocksdb wrapper version 0.3

Conflicts:
	src/uint.rs

from_existing works for an empty DB if passed empty RLP SHA3; Option types in JSON.

New JSON conversion traits.