Track the per-page contribution to the cache's dirty byte count.  If this is non-zero when a page is being discarded, fix the cache's count.

refs #635, #699
Merge pull request #708 from wiredtiger/cache-dirty-bytes2

Track the per-page contribution to the cache's dirty byte count.
Merge branch 'develop' into cache-dirty-bytes
Cleanup after merge: remove some duplicate code / comments.
Merge branch 'cache-dirty-bytes' into tighter-spinlocks