core: introduce grace period

In report mode we break out from select loop immediately after we reach
maximum count of iterations. But we should wait for packets which are still on
the way.

In order to fix the issue we introduce grace period during which we don't send
out more packets but we just wait for responses which might be still on the way.

resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1009051