Support snapshots in LSM via update checks, rather than actual updates.

The current code inserts updates into old chunks, in order to check for
conflicts with snapshot transactions. It's enough to do the conflict check
without actually updating, and doing a check saves some mess with tracking
when it's OK to checkpoint non-primary chunks.
@michaelcahill I moved this check from ```__wt_row_modify``` because I didn't like that we were potentially unnecessarily creating modify structures. Moving it here makes the check specific to insert (not update), so I'm happy for it to move back if you think it's a better fit.