net: lwm2m: Implement fallback mechanism and support for diable

If server registration fails, allow fallback to secondary server,
or fallback to bootstrap.
Also allow fallback to different bootstrap server.

Add API to tell RD client when server have been disabled by
executable command.

Changes to RD state machine:
* All retry logic should be handled in NETWORK_ERROR state.
* New state SERVER_DISABLED.
* Internally disable servers that reject registration
* Temporary disable server on network error.
* Clean up all "disable timers" on start.
* Select server first, then find security object for it.
* State functions return void, error handling is done using states.
* DISCONNECT event will only come when client is requested to stop.
* NETWORK_ERROR will stop engine. This is generic error for all kinds
  of registration or network failures.
* BOOTSTRAP_REG_FAILURE also stops engine. This is fatal, and we cannot
  recover.

Refactoring:
* Server selection logic is inside server object.
* sm_handle_timeout_state() does not require msg parameter. Unused.
* When bootstrap fail, we should NOT back off to registration.
  This is a fatal error, and it stops the engine and informs application.

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