WiredTiger connection close can hang joining async threads.

Currently, WiredTiger clears the WT_CONN_SERVER_ASYNC flag before attempting to join all of the async threads, which should exit their run loop as the above flag has been cleared.

It appears that there is a potential situation where the WT_CONN_SERVER_ASYNC flag can be reset following a loop through capturing statistics as the flag is set in the __wt_async_stats_update function.

From a rudimentary review, I don't believe we need to set the flag in __wt_async_stats_update so we should remove the call from there to avoid this hang.