Add a reference count to LSM chunks, so that chunks can't be freed
while worker threads are accessing them.
Write lock around refcnt changes.
Don't decrement LSM chunks that have been free'd.
Fix a bug in LSM chunk reference counting on shutdown.
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.
Merge branch 'develop' into lsm_worker_refcount
Merge pull request #696 from wiredtiger/lsm_worker_refcount

Add a reference count to LSM chunks
lint