tests: lwm2m: Implement event stream support for Leshan

Implement support for reading stream of events from Leshan.
This allows testing the LwM2M SEND/Notify/Update operations
and reading content of those.

Also convert the whole Leshan class to use requests.session() so
it pools up connections and uses keep-alive.

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