Remove ReadOnlyStateDB, use snapshot+revert

I has been stated that StaticCall can do modifications to the state in
multiple places in the geth code base. Therefore, we first did a copy of
the state to avoid changes and later switched to used the
ReadOnlyStateDB to achieve the same result with better performance.

Upon closer examination, the only state change that StaticCall actually
does is "touching" the called address, which only makes a difference on
legacy Ethereum chains, and tracking the access list. This can be
avoided by doing a snapshot+revert. The performance impact should be
low, since the revert only has to undo the access list change.

With the removal of ReadOnlyStateDB, further testing of it is pointless:
Closes celo-org/celo-blockchain-planning#355

contracts/celo_backend.go

@@ -48,9 +48,13 @@ func (b *CeloBackend) CallContract(ctx context.Context, call ethereum.CallMsg, b
 	txCtx := vm.TxContext{}
 	vmConfig := vm.Config{}
 
-	readOnlyStateDB := ReadOnlyStateDB{StateDB: b.State}
-	evm := vm.NewEVM(blockCtx, txCtx, &readOnlyStateDB, b.ChainConfig, vmConfig)
+	// While StaticCall does not actually change state, it changes the
+	// access lists. We don't want this to add any access list changes, so
+	// we do a snapshot+revert.
+	snapshot := b.State.Snapshot()
+	evm := vm.NewEVM(blockCtx, txCtx, b.State, b.ChainConfig, vmConfig)
 	ret, _, err := evm.StaticCall(vm.AccountRef(evm.Origin), *call.To, call.Data, call.Gas)
+	b.State.RevertToSnapshot(snapshot)
 
 	return ret, err
 }