Enhance forced eviction to allow multiple pages to be queued.
Increment the data source cursor count in the cursor layer (where we know
we have a data source handle, currently, where we know we have a btree
handle).
WT_DSTAT_DECR should have been changed from btree stats to dhandle
stats.
Merge pull request #496 from wiredtiger/multi-forced-eviction

Enhance forced eviction to allow multiple pages to be queued.