Custom unbounded readers are read in bundles of at most
10k elements or 10 seconds. A recent change accidentally removed
the 10k element limit. This change reintroduces it and
adds a test.

The previous test also was passing vacuously because
the iteration limit was incorrect (it would always
have only one iteration).
----Release Notes----
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=112723469