In diagnostic mode, assert instead of doing the panic, it makes it easier
to figure out what thread went wrong.
Try to fix the two problems with the original overflow work in the
new-split code (#1: removed overflow objects don't stay in the cache
past their useful life, so they may not exist in the cache when we try
and get them, #2: we don't want to write overflow objects that aren't
useful to any possible reader anyway).

The plan is to write the WT_CELL_VALUE_OVFL_RM cells into the page's
disk image, and then when the page is re-instantiated in memory, it's
checked for those cells.  If we find those cells, eviction fixes up the
references by copying removed overflow objects from the original page's
reconciliation cache into the newly instantiated page's reconciliation
cache if they exist, and ignoring them if they don't (if they don't
exist, no possible reader can care).
Merge pull request #916 from wiredtiger/new-split-overflow-2

Instead of writing overflow records, figure it out when re-instantiating the page.