modules/afmongodb: Update the embedded libmongo-client copy

The libmongo-client copy was accidentally reverted to an older version
some time ago, update it to the latest stable release again.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
Preparations for 3.4.5

NEWS & VERSION updated for syslog-ng 3.4.5.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>