distributor: fix livelock on flush

distributor: fix livelock on flush

The Distributor autotest can lock if ran enough times. Worker and
distributor threads get into a livelock situation waiting on each
other.

To repeat:
`while sudo sh -c "echo 'distributor_autotest' |
		./build/app/test/dpdk-test"; do :; done`

The root cause is where we are flushing on exit, and do not wait for
all worker packets to be returned before exiting.
Add a delay on flush so that all worker packets are returned before
completing the flush.

Bugzilla ID: 316
Fixes: 775003ad2f96 ("distributor: add new burst-capable library")
Cc: stable@dpdk.org
Reported-by: Michael Santana <msantana@redhat.com>
Signed-off-by: David Hunt <david.hunt@intel.com>
Signed-off-by: Liang Ma <liang.j.ma@intel.com>
Tested-by: Michael Santana <msantana@redhat.com>