net: lwm2m: Decouple CoAP message- and block size

If blockwise transfer is not in use it must be possible to send bigger
CoAP messages than what is the block size used with blockwise transfers.

To compensate for the increased memory usage number of in-flight packets
allowed could be decreased.

Signed-off-by: Veijo Pesonen <veijo.pesonen@nordicsemi.no>