rgw: d3n: fix valgrind reported leak related to libaio worker threads

rgw: d3n: fix valgrind reported leak related to libaio worker threads

which sporadically reproduces on teuthology ubuntu instances

happens because a race between RGW shutdown occurring before
the libaio worker threads had terminated

to fix, reduced the libaio threads inactivity shutdown time

ref:
man aio_init
...
aio_idle_time
This field specifies the amount of time in seconds that a worker thread
should wait for further requests before terminating, after having
completed a previous request.   The
default value is 1.
...

Fixes: https://tracker.ceph.com/issues/63445
Signed-off-by: Mark Kogan <mkogan@ibm.com>