Example of nested attribute access:
```
a.b.c.d
```

Before this commit, only the outermost object was properly locked when
accessing its attribute. For the inner objects which are themselves
attributes of outermore objects, the locks were released immediately
after being acquired and before accessing the object's attribute.

Pseudo code example:
```
lock a
temp_b = a.b
lock temp_b
unlock temp_b
temp_c = temp_b.c
lock temp_c
unlock temp_c
temp_c.d
unlock a
```

instead of:
```
lock a
temp_b = a.b
lock temp_b
temp_c = temp_b.c
lock temp_c
temp_c.d
unlock temp_c
unlock temp_b
unlock a
```