core/state: implement reverts by journaling all changes

This commit replaces the deep-copy based state revert mechanism with a
linear complexity journal. This commit also hides several internal
StateDB methods to limit the number of ways in which calling code can
use the journal incorrectly.

As usual consultation and bug fixes to the initial implementation were
provided by @karalabe, @obscuren and @Arachnid. Thank you!

core/chain_makers.go

@@ -131,7 +131,7 @@ func (b *BlockGen) AddUncheckedReceipt(receipt *types.Receipt) {
 // TxNonce returns the next valid transaction nonce for the
 // account at addr. It panics if the account does not exist.
 func (b *BlockGen) TxNonce(addr common.Address) uint64 {
-	if !b.statedb.HasAccount(addr) {
+	if !b.statedb.Exist(addr) {
 		panic("account does not exist")
 	}
 	return b.statedb.GetNonce(addr)