Use the eviction server to write pages with READGEN_OLDEST set.

Even before the eviction trigger has been reached. This should mean
that we clear those pages out of cache earlier, and hopefully will
save application threads from doing the evictions (at least sometimes).
Check that handles are not being walked by eviction before discarding.

refs #1497
Merge branch 'develop' into evict-trickle
Only trigger the eviction server to look for pages that would force eviction if regular eviction either fails or does an in-memory split.
Eviction should do update-restore if upper layers are trying to force out a page, regardless of its size.  Also, only look at ref->page after checking for exclusive access.  It is possible (but very unlikely) that a child page pointer could be replaced in the window where we are checking hazard pointers.
Rename flags to call this phase "eviction of pages that would block application threads".
Merge pull request #1500 from wiredtiger/evict-trickle

Use the eviction server to write pages with READGEN_OLDEST set.