Switch to using hazard references for walks through the cache.
Remove the code associated with WT_REF_EVICT_WALK.
Merge branch 'develop' into evict-walk-hazard
Update a couple of comments.
Remove the comments in __wt_bt_cache_op(), they're wrong and it makes
more sense for the discussion to be in __wt_sync_file, anyway.

Remove the WT_PUBLISH of the WT_BTREE.ckpt, it's no longer necessary
because we're not making a request of the eviction server thread.
Rework the initial comment describing different walks.
Set the WT_READ_NO_BUMP flag for compaction, compaction should make a
page more valuable.
whitespace
There's only one remaining verbose hazard pointer message (on
acquisition, we no longer have one on hazard pointer release).
Remove the verbose hazard message, we don't need another test
in the hazard pointer path, and hazard pointer acquisition is
too verbose to be useful anyway.
Fix a comment, there's no "page server" thread.
Merge pull request #886 from wiredtiger/__wt_bt_cache_op

evict-walk-hazard changes
Merge branch 'develop' into evict-walk-hazard
Merge branch 'develop' into evict-walk-hazard
[mq]: evict-walk-review
Make a flag so that avoiding forced eviction is explicit, and always on for the eviction server thread.
We already have a per-tree flag to disable unexpected eviction, use that in preference to a session flag.
Merge pull request #885 from wiredtiger/evict-walk-hazard

Use hazard pointers for eviction walks.
Fix a warning from using a variable called "truncate":
../src/btree/bt_curnext.c: In function '__wt_btcur_next':
../src/btree/bt_curnext.c:374:43: error: declaration of 'truncate' shadows a global declaration [-Werror=shadow]
../src/btree/bt_curprev.c: In function '__wt_btcur_prev':
../src/btree/bt_curprev.c:466:43: error: declaration of 'truncate' shadows a global declaration [-Werror=shadow]