Refactor state into refs and tree.

[silasdavis]

This prepares the way for various state manipulations and introduces a top-level storage package that takes makes some steps towards Cosmos SDK compatibility. Changes included:

• Prefix and KeyFormat types for handling prefixed lexicographic keys consistently
• A CacheDB that wraps all changes within a block into a single DB transaction - i.e. Batch
• A separate 'tree' and 'refs' storage in state. The former describes those values that contribute to the state hash and the latter does not - this gives us more latitude in disabling, for example, block storage or running nodes that opt to store somewhat different derived data whilst agreeing the the core account state.
• Upgrade IAVL dependency to version including: Use 8 bytes to store int64 components of database keys  cosmos/iavl#107

Some changes that may follow in a future PR:

• Content address account code for some deduplication
• Content address blocks to allow their hash to be stored in main tree for consistency (not sure about this one)
• Make block storage configurable
• Consider using a Trie implementation in various places including caches for some reduced string -> []byte copying.

[seanyoung]

storage/cache_db.go line 45 contains a bug statement.
as @silasdavis is away let's merge anyway and I will fix up afterwards.