When node is being reset, the init functions of each sig-defined
models will be invoked again, this will cause memory leak because
some model internal data will be allocated again.
Hence before trying to allocate memory for them, we add some check
to make sure no memory has been allocated previously.
And for client model, when the init functions are invoked again,
we will clear the list items.
Creates macros for determining model message lengths based on opcode,
payload length and MIC size. Also adds macro wrapping
NET_BUF_SIMPLE_DEFINE to serve the most common use case.
