net: lwm2m: Refactor blockwise SEND to support GET and FETCH

Allow blockwise-send buffers to be used with GET and FETCH
queries as well.
When outgoing packet is split into multiple blocks, don't free
it when first block is send. Keep it in memory until some other requests
come.

Following queries to next block are matched using CoAP token.
However, this required Leshan to use COAP.BLOCKWISE_REUSE_TOKEN=true
option from Californium.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>