problem: concurrent rw on state db stateobjects map

solution: use mutex lock

fixes #284, again

core/state/statedb.go

@@ -349,12 +349,15 @@ func (self *StateDB) deleteStateObject(stateObject *StateObject) {
 // Retrieve a state object given my the address. Returns nil if not found.
 func (self *StateDB) getStateObject(addr common.Address) (stateObject *StateObject) {
 	// Prefer 'live' objects.
+	self.lock.Lock()
 	if obj := self.stateObjects[addr]; obj != nil {
+		self.lock.Unlock()
 		if obj.deleted {
 			return nil
 		}
 		return obj
 	}
+	self.lock.Unlock()
 
 	// Load the object from the database.
 	enc, err := self.trie.TryGet(addr[:])