net: ipv6: Improve Neighbor Discovery thread safety

Currently, the only thread-safe part of the IPv6 Neighbor processing
implementation are stale_counter related operation.

Fix this, by extending the mutex protection over all of the module, so
that message handlers, timers and API functions do not interfere with
each other.

As IPv6 Neighbor cache is tightly coupled with the Routing module, use
the same mutex to protect both, neighbor and routing tables, to prevent
deadlocks.

Also, replace the semaphore used with a mutex, as it seems more fit for
this particular job.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>