- removed assertion that checks whether persistent bats have at least one logical refcnt,   because it does not necessarily hold during BATmode(b,TRANSIENT).   I.e. BATmode() first decreases the reference, and only afterwards changes the   batPersistence field from PERSISTENT to TRANSIENT.
- removed assertion that checks whether persistent bats have at least one logical refcnt,
  because it does not necessarily hold during BATmode(b,TRANSIENT).
  I.e. BATmode() first decreases the reference, and only afterwards changes the
  batPersistence field from PERSISTENT to TRANSIENT.

  This does not show in most MIL code because many constructs like passing
  the BAT in a proc or getting it with a bat(str) cast add a temporary
  logical reference (so the assertion does not go off).

  Roberto bumped into it when using the "destroy" commando.

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