One more idea - is just forbid HOT prune while the second phase is
running. It is not possible anyway currently because of snapshot held.
Possible enhancements:
* we may apply restriction only to particular tables
* we may apply restrictions only to part of the tables (not yet
scanned by R/CICs).
Yes, it is not an elegant solution, limited, not reliable in terms of
architecture, but a simple one.
