[mle] maintain existing child id whenever possible (#2101)
[mle] maintain existing child id whenever possible (#2101)

Child IDs are assigned when receiving a Child ID Request.  Prior to this
commit, a new Child ID is assigned whenever the child state is not valid.
A router stores state about its attached children in non-volatile memory.
On a reboot, the router attempts to restore its child links.  As a result,
it is possible for the child to be assigned a new Child ID while it is
being restored.

With this commit, a router only assigns a new Child ID if there is no
existing Child ID state for the given child.
pvanhorn/openthread,erja-gp/openthread,openthread/openthread,pvanhorn/openthread,bukepo/openthread,chshu/openthread,vaas-krish/openthread,fbsder/openthread,georgecpr/openthread,vaas-krish/openthread,lanyuwen/openthread,fbsder/openthread,mszczodrak/openthread,chshu/openthread,aeliot/openthread,fbsder/openthread,abtink/openthread,LeZhang2016/openthread,georgecpr/openthread,lanyuwen/openthread,chshu/openthread,librasungirl/openthread,gandreello/openthread,mszczodrak/openthread,turon/openthread,georgecpr/openthread,srickardti/openthread,gandreello/openthread,chshu/openthread,aeliot/openthread,xiaom-GitHub/openthread,xiaom-GitHub/openthread,aeliot/openthread,abtink/openthread,openthread/openthread,georgecpr/openthread,fbsder/openthread,erja-gp/openthread,gandreello/openthread,mszczodrak/openthread,pvanhorn/openthread,lanyuwen/openthread,aeliot/openthread,vaas-krish/openthread,LeZhang2016/openthread,LeZhang2016/openthread,LeZhang2016/openthread,jwhui/openthread,mszczodrak/openthread,bukepo/openthread,georgecpr/openthread,xiaom-GitHub/openthread,openthread/openthread,mszczodrak/openthread,mszczodrak/openthread,srickardti/openthread,aeliot/openthread,LeZhang2016/openthread,chshu/openthread,turon/openthread,srickardti/openthread,abtink/openthread,openthread/openthread,erja-gp/openthread,turon/openthread,erja-gp/openthread,pvanhorn/openthread,pvanhorn/openthread,xiaom-GitHub/openthread,jwhui/openthread,jwhui/openthread,xiaom-GitHub/openthread,vaas-krish/openthread,librasungirl/openthread,gandreello/openthread,librasungirl/openthread,xiaom-GitHub/openthread,lanyuwen/openthread,pvanhorn/openthread,LeZhang2016/openthread,lanyuwen/openthread,georgecpr/openthread,srickardti/openthread,gandreello/openthread,jwhui/openthread,vaas-krish/openthread,bukepo/openthread,gandreello/openthread,erja-gp/openthread,aeliot/openthread,fbsder/openthread,turon/openthread,bukepo/openthread,erja-gp/openthread,lanyuwen/openthread,chshu/openthread,vaas-krish/openthread,librasungirl/openthread,abtink/openthread,fbsder/openthread