Move WT_PAGE.bytes_dirty into WT_PAGE_MODIFY.bytes_dirty, there won't be
dirty bytes to track unless the page has been modified, and it saves 8B
on read-only pages.

Move bytes_dirty resolution from __wt_page_out() to __wt_cache_page_evict(),
we're updating up our cache information there anyway, it makes more sense.
Add __wt_cache_dirty_incr() to match __wt_cache_dirty_decr(), and make
them look the same.