#3405 mds: mds agent raise coredump when it terminates by calling exit

[Nguyen Huynh Tai]

An MDS agent process that terminates by calling exit() while flow-control
threads are still running can produce a coredump.

exit() destroys the flow-control objects at static-destructor time but does
not stop the still-running flow-control and worker threads. A surviving
thread then accesses that already-destroyed objects, causing a
use-after-free that crashes the process