Don't stream by default

Current `_Iterator` streams data from the server by default. This
behavior, although useful, shouldn't be considered a sane default. A
good example is when using `limit` on listing operations. If the default
is to stream, the limit will be ignored because the server client will
get everything from the server until there's anything left to consume.
This behavior can also be harmful if not used carefully.

This patch adds a `stream` method to the iterator and makes using this
behavior an explicit operation. The user has to opt-in for data
streaming.

Change-Id: Ib1af24960dff97cb956990d4caf88705f2f7a0d5
Merge "Don't stream by default"