Prevent the server crash by ensuring the backup cursor is closed before the server shutdown

Hi,

PSMDB crashes on shutdown if data-at-rest encryption is enabled and backup cursor isn't closed before shutdown.

STR:

1) Start PSMDB with data-at-rest enabled

2) Open backup cursor 

3) Send SIGTERM signal to the process

Actual result: PSMDB crash

Expected result: clean PSMDB shutdown 

Full log is attached