Potential race in __sweep_discard_trees

In  __sweep_expire_one we are holding the dhandle->rwlock while we call __wt_conn_dhandle_close. In __sweep_discard_trees, we do not hold a lock while making the same call.  We should understand the reasons for holding the lock in the first case, and consider holding the same lock.  sulabh.mahajan noticed this issue in the work for WT-7392.