net: lwm2m: Synchronized functions mutating the RD client's state

A mutex is used to syncrhonize the start, stop and service() functions
of the RD client. Previously it could happen that while service() was
working on e.g. bootstrapping, a stop() call by another thread would
close the socket. Then the bootstrapping process would detect it as a
network error, and restart the process.

Fixes #37170.

Signed-off-by: Maik Vermeulen <maik.vermeulen@innotractor.com>