test/format failure: checkpoint ranges never verified

This issue is reproduced with the following configuration:

```
############################################
#  RUN PARAMETERS
############################################
# bitcnt not applicable to this run
cache=30
compression=none
data_extend=0
data_source=table
delete_pct=7
dictionary=1
file_type=row-store
hot_backups=0
huffman_key=0
huffman_value=0
insert_pct=50
internal_key_truncation=0
internal_page_max=13
key_gap=16
key_max=64
key_min=12
leaf_page_max=12
ops=500000
prefix=1
repeat_data_pct=30
reverse=0
rows=100
runs=0
split_pct=52
threads=20
value_max=256
value_min=19
# wiredtiger_config not applicable to this run
write_pct=45
```
The configuration is very similar to that in issue #639, but the symptoms are different. I've seen two types of failure:

```
  10: table, row-store                                                        
t, file:wt.wt, session.checkpoint: illegal cell and page type combination: cell 4 on page at [write-check] is a value/ovfl,rm cell on a row-store leaf page
t: session.checkpoint: WT_ERROR: non-specific WiredTiger error
```
and:
```
  12: table, row-store                                                        
t, file:wt.wt, session.verify: checkpoint ranges never verified: 15       
t, file:wt.wt, session.verify: file ranges never verified: 669
t: session.verify: table:wt: post-ops verify: WT_ERROR: non-specific WiredTiger error
```

Running on a machine with SSDs, the failure generally happens after less than 20 iterations of test/format.

If I disable the in-memory page split code, I no longer see a failure.