whitelist: Use a watcher for control socket reading rather than blocking

whitelist: Use a watcher for control socket reading rather than blocking

Performing a stream read_all call (which is a blocking read) from
within the accept callback has the issue that if a whitelist client is
still connected whilst a shutdown of the charon deamon is triggered
then that shutdown won't complete gracefully due to the accept task
never exiting.

So fix shutting down gracefully by using the socket watcher rather than
a blocking read upon connection accept. Fall back to a blocking read
for partial messages to avoid the complexity associated (i.e. storing
state) for incomplete reads, which shouldn't block and cause the
original problem if the client only sends whole messages.