dnsdist: Fix a race in the XSK/AF_XDP backend handling code

dnsdist: Fix a race in the XSK/AF_XDP backend handling code

For performance reasons we used to keep a local list of available frames
in our `XskWorker` object, like we are doing in the `XskSocket` one,
to avoid having to go to the shared list which is protected by a lock.
Unfortunately, while it works well for the `XskSocket` because it is
accessed by a single `XskRouter` thread, the `XskWorker` object can
be accessed by multiple threads at once: `XskResponderThread`,
`responderThread`, `XskClientThread` and `XskRouter`. Most of the
time these threads do not acquire nor release frames to the local
list, but `responderThread` does acquire one when a response frame
is punted to the regular networking stack, and all of them release
frames when an unexpected condition occurs, for example when a queue
is full. This leads to memory corruption and to a crash.

This commit gets rid of the local list of frames in the `XskWorker`
object, acquiring and releasing them to the shared list instead, since
performance in these cases is likely not as critical. If it turns out
to be too slow, we can look into caching a few frames in a thread-local
list, but then we need to be careful not to hold on them indefinitely
which might be tricky.