Revert "Allow batch_size=None in Table.scan() to avoid filter incompatibilities"

This reverts commit 8481d317804e060b12839b571ef22306074fba9c, since it
is not a correct fix. See issue #56.
No longer confuse batching/caching; add Table.scan(scan_batching=...)

For details, see the comments added in this commit, and issues #54 and
issue #56.