Remove __wt_eviction_force entirely, just set the page read-gen in
__wt_eviction_force_check, the next page release will find it (and
change the page-in function to call __wt_page_release).

Remove the WT_PAGE_EVICT_FORCE flag, all it does is wake up the
eviction server, once per forced eviction page, and there are
lots of other paths to wake the eviction server if it's needed.
Merge pull request #907 from wiredtiger/new-split-2

more simplification of forced-eviction from page-in function