LSM search has to look up a key's value when it does non-overwrite inserts
and/or upates because it's looking for deleted records and that's flagged
in the value.  Don't overwrite the application's WT_ITEM value and avoid
any possibility of leaking memory.  Reference #965.
Simplify the change to not overwrite &cursor->value in LSM lookups.

refs #966
Merge pull request #966 from wiredtiger/lsm-use-local-value

overwriting/leaking the application's WT_ITEM value