Some fixes for in-memory splits:

1. wait until we have checked that we have exclusive access to the page;
2. don't try to evict the page inline: leave it for an eviction thread;
3. make a copy of WT_REF.ikey: we need the original and the new one to survive the split; and
4. only look at WT_REF.ikey in row stores.

refs #1419