Fix memory leak when breaking mid-way in _get_objects and _get_ids

[akariv]

This PR fixes a memory leak that is caused from resources not being freed when using _get_objects and _get_ids and not consuming all of the items in the generator.

Explanation - this is the way these functions are implemented currently:

>>> def objects():
...   try:
...     for i in range(10):
...       yield i
...     print('release mem 1')
...   except Exception as e:
...     print('release mem 2')
...     raise

When iterating the objects, but breaking mid way, the memory is not released:

>>> for x in objects():
...   print('got', x)
...   if x == 5: break
... 
got 0
got 1
got 2
got 3
got 4
got 5
>>> 

If we use finally instead, it simplifies the code and ensures that memory is always released:

>>> def objects():
...   try:
...     for i in range(10):
...       yield i
...   finally:
...     print('release mem 3')

This works when consuming the entire iterator:

>>> for x in objects():
...   print('got', x)
... 
got 0
got 1
...
got 8
got 9
release mem 3
>>> 

And when breaking in the middle:

>>> for x in objects():
...   print('got', x)
...   if x == 5: break
... 
got 0
got 1
got 2
got 3
got 4
got 5
release mem 3

[adamjstewart]

This seems correct to me but I'll let @hobu or someone else review too.