Sweep eviction improvements

Improve interactions between sweep and eviction by not holding the handle list lock while walking files.
It's a performance kludge.  It doesn't actually result in any walks being cleared: all it does is have eviction break out of the walk for new pages.  The "clear walk" function now only applies to the trees that a session publishes as waiting for the walk to be cleared.

So in this case, the eviction server will give up the data handle lock (allowing sweep to make progress), then do a scan of sessions and find that none of them need a tree walk to be cleared.

It's definitely a kludge, but it doesn't actually give up all of the eviction state.