samples: net: sockets: http_server: Dont block system workq

Since the netstats handler calls on functions which reach deep into the
networking stack, there is a lot of points actually at which it can be
blocked, even forever. So having this handler on the system workqueue is
not a good idea and can even cause a deadlock in some cases if it's
blocked waiting on a synchronization primitive that would be given by a
work item scheduled later in the queue. Therefore, make a workqueue
specifically for this http server socket instead of using the system
one.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>