When discarding pages for a file, clear the tree's root page reference
at the root page discard in the WT_SYNC_DISCARD case, as well as in the
WT_SYNC_DISCARD_NOWRITE case.

Assert we never discard a dirty/modified page (requires changes to
WT_SYNC_DISCARD_NOWRITE to mark the page clean before discarding it).

When evicting a page with unresolved changes, the page won't have been
marked clean by reconciliation, do it explicitly.