cache: fix race in assert_right_version

This change fixes race condition in assert_right_version(). Racy
situation:
- Two instances have the (empty) cache open: New binary and old binary.
- New binary executes count() inside assert_right_version(), which
  internally starts RO transaction. Returned count is 0.
- Old binary does some writes (RW transaction parallel to RO in the first
  process).
- New binary skips cache clear because cache was empty at the time of check.
- Result: The old binary wrote data with an old format into cache which
  was not cleared and silenty changed version number to a new one.

This is not complete fix because we lack mechanism to detect cache format
change at run-time, but at least it removes one nasty corner case and
cost of this change seems to be minimal.