read_view: add space upgrade function to space_read_view

There may be a space upgrade in progress at the time when a read view is
created. In this case, we should apply the upgrade function to tuples
retrieved from the space read view. We can't just use the space upgrade
function as is, because it may be dropped while the read view is still
in use. So we need to create a special upgrade function for the read
view. This commit adds stubs for this, which will be implemented in the
EE repository.

Needed for https://github.com/tarantool/tarantool-ee/issues/163

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NO_TEST=ee
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