Fix a comment.
Fix a bug where last_running was used instead of __wt_txn_visible_all,
the latter is correct.

Fix a bug where we weren't locking the WT_OVFL_TXNC structure on wrapup,
we have to lock out readers if we're going to clean up the list.  Since
we call wrapup regardless of there actually being items on the overflow
lists, check the lowest level of each skiplist before doing any work at
all -- this change applies to all 3 lists, might as well save the work.