net: lwm2m: Make LwM2M message allocation thread safe

The LwM2M message allocation was not thread safe, i. e. the message was
acquired by setting the ctx pointer, and it was freed by clearing the
entire message structure.

If preemptive threads were enabled, and memset() clearing the message
content was preempted, the message structure being currently zeroed
migh've been allocated and initialized by some other thread. If the
thread releasing the message resumed work, it would continue clearing
the freshly allocated and initialized message structure.

In order to prevent this, introduce a new global mutex for the lwm2m
engine global variables. The mutex is used when LwM2M message is
allocated/deallocated. This will prevent reallocating the message
during the release process.

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