Semantic sugar.

Inline __wt_cache_bytes_dirty() and __wt_cache_pages_dirty(), there's no
policy of not directly accessing WT_CACHE fields, and that's all they do.

Remove comments about 8B vs. 4B atomic read/write and "special" code in
__wt_cache_pages_inuse() and __wt_cache_bytes_inuse(), WiredTiger
requires 8B atomic read/write, the comments no longer make any sense.