net: lwm2m: replace periodic service thread with work queue

We can save some resources by removing the periodic service thread
and replacing it by queuing the services to the work queue.

Before (reel_board using BT + DTLS)
Memory region         Used Size  Region Size  %age Used
           FLASH:      289464 B         1 MB     27.61%
            SRAM:       75620 B       256 KB     28.85%
        IDT_LIST:         136 B         2 KB      6.64%

After
Memory region         Used Size  Region Size  %age Used
           FLASH:      289576 B         1 MB     27.62%
            SRAM:       74596 B       256 KB     28.46%
        IDT_LIST:         136 B         2 KB      6.64%

Signed-off-by: Michael Scott <mike@foundries.io>