net: lwm2m: Create socketpair that can wake up zsock_poll()

Allow socket-loop to wake up immediately, if there are changes,
instead of waiting for zsock_poll() to timeout.
This change makes engine more reactive and removes
hard coded timeout from zsock_poll().

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