add docs for playing nice with couchdb

I assume this applies more to one-off stuff like feature migrations and management commands than to day-to-day usage?  Eg: I'm changing the way X is stored on users, so I need to go through all users, make an edit, then save.  Rather than:
```python
for user_id in big_list_of_user_ids:
    user = CommCareUser.get(user_id)
    transform(user)
    user.save()
```
I should:
``` python
users_to_save = []
for raw_user in iter_docs(db, big_list_of_user_ids):
    user = CommCareUser(raw_user)  # I'm pretty sure there's a better way to wrap iter_docs results
    transform(user)
    users_to_save.append(user)
    if len(users_to_save) == 1000:
        bulk_save(users_to_save)
        users_to_save = []
```
And query the view in between the bulk saves even?

Do we have an abstraction for bulk saving analagous to iter_docs?  Sort of a reverse generator?  Could be a useful util to have around.  I'd be interested in making one if it doesn't exist.  Maybe something like:
``` python
with BulkSaver() as bulk_saver:
    for ...:
        bulk_saver.save(doc)
```