[ncp] reset iterator in get handler of OFF_MESH_ROUTES for local network data (#2161)
[ncp] reset iterator in get handler of OFF_MESH_ROUTES for local network data (#2161)

This commit also simplifies the implementation for `ON_MESH_NETS`
get handler.
aeliot/openthread,georgecpr/openthread,lanyuwen/openthread,fbsder/openthread,pvanhorn/openthread,pvanhorn/openthread,mszczodrak/openthread,xiaom-GitHub/openthread,LeZhang2016/openthread,bukepo/openthread,fbsder/openthread,jwhui/openthread,mszczodrak/openthread,lanyuwen/openthread,gandreello/openthread,erja-gp/openthread,georgecpr/openthread,xiaom-GitHub/openthread,turon/openthread,chshu/openthread,vaas-krish/openthread,LeZhang2016/openthread,jwhui/openthread,chshu/openthread,abtink/openthread,chshu/openthread,aeliot/openthread,georgecpr/openthread,turon/openthread,mszczodrak/openthread,lanyuwen/openthread,aeliot/openthread,gandreello/openthread,vaas-krish/openthread,aeliot/openthread,abtink/openthread,pvanhorn/openthread,bukepo/openthread,LeZhang2016/openthread,srickardti/openthread,librasungirl/openthread,aeliot/openthread,lanyuwen/openthread,gandreello/openthread,mszczodrak/openthread,gandreello/openthread,chshu/openthread,vaas-krish/openthread,pvanhorn/openthread,turon/openthread,fbsder/openthread,georgecpr/openthread,georgecpr/openthread,chshu/openthread,librasungirl/openthread,xiaom-GitHub/openthread,mszczodrak/openthread,LeZhang2016/openthread,erja-gp/openthread,xiaom-GitHub/openthread,turon/openthread,srickardti/openthread,gandreello/openthread,mszczodrak/openthread,librasungirl/openthread,vaas-krish/openthread,LeZhang2016/openthread,fbsder/openthread,openthread/openthread,bukepo/openthread,pvanhorn/openthread,gandreello/openthread,openthread/openthread,lanyuwen/openthread,LeZhang2016/openthread,bukepo/openthread,srickardti/openthread,georgecpr/openthread,openthread/openthread,lanyuwen/openthread,erja-gp/openthread,aeliot/openthread,erja-gp/openthread,librasungirl/openthread,fbsder/openthread,pvanhorn/openthread,jwhui/openthread,erja-gp/openthread,fbsder/openthread,abtink/openthread,xiaom-GitHub/openthread,srickardti/openthread,abtink/openthread,erja-gp/openthread,xiaom-GitHub/openthread,chshu/openthread,vaas-krish/openthread,vaas-krish/openthread,openthread/openthread,jwhui/openthread