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.
Merge branch 'develop' into lsm_worker_refcount
Merge pull request #696 from wiredtiger/lsm_worker_refcount

Add a reference count to LSM chunks