A race in eviction determining whether an internal page is dirty

There was a test/format failure in Jenkins that triggered the following assertion:

t, file:wt, eviction-worker: ../src/btree/bt_discard.c, 57: !__wt_page_is_modified(page)

i.e: That a page is clean when we are attempting to discard it. The particular page is an internal page and all of it's children are marked deleted.

Failure:
http://build.wiredtiger.com:8080/job/wiredtiger-test-format-stress/9003/