[mle] reset last heard on receiving MLE Parent Request (#1991)
[mle] reset last heard on receiving MLE Parent Request (#1991)

When a child is not attached, set the last heard and timeout values on
receiving an MLE Parent Request message.  This ensures that the child
state is not prematurely timed out when a child is trying to attach.
librasungirl/openthread,vaas-krish/openthread,xiaom-GitHub/openthread,LeZhang2016/openthread,srickardti/openthread,lanyuwen/openthread,georgecpr/openthread,gandreello/openthread,fbsder/openthread,JakubBrachTieto/openthread,openthread/openthread,bukepo/openthread,pvanhorn/openthread,librasungirl/openthread,bukepo/openthread,JakubBrachTieto/openthread,fbsder/openthread,xiaom-GitHub/openthread,xiaom-GitHub/openthread,erja-gp/openthread,fbsder/openthread,abtink/openthread,librasungirl/openthread,georgecpr/openthread,lanyuwen/openthread,erja-gp/openthread,LeZhang2016/openthread,LeZhang2016/openthread,georgecpr/openthread,srickardti/openthread,pvanhorn/openthread,abtink/openthread,gandreello/openthread,turon/openthread,openthread/openthread,turon/openthread,mszczodrak/openthread,vaas-krish/openthread,lanyuwen/openthread,turon/openthread,vaas-krish/openthread,erja-gp/openthread,chshu/openthread,chshu/openthread,abtink/openthread,chshu/openthread,jwhui/openthread,LeZhang2016/openthread,librasungirl/openthread,chshu/openthread,LeZhang2016/openthread,chshu/openthread,xiaom-GitHub/openthread,erja-gp/openthread,aeliot/openthread,georgecpr/openthread,aeliot/openthread,gandreello/openthread,lanyuwen/openthread,chshu/openthread,jwhui/openthread,openthread/openthread,bukepo/openthread,lanyuwen/openthread,bukepo/openthread,mszczodrak/openthread,aeliot/openthread,fbsder/openthread,vaas-krish/openthread,gandreello/openthread,mszczodrak/openthread,abtink/openthread,pvanhorn/openthread,georgecpr/openthread,xiaom-GitHub/openthread,fbsder/openthread,mszczodrak/openthread,pvanhorn/openthread,srickardti/openthread,openthread/openthread,pvanhorn/openthread,jwhui/openthread,vaas-krish/openthread,georgecpr/openthread,JakubBrachTieto/openthread,mszczodrak/openthread,xiaom-GitHub/openthread,fbsder/openthread,LeZhang2016/openthread,gandreello/openthread,vaas-krish/openthread,erja-gp/openthread,jwhui/openthread,erja-gp/openthread,aeliot/openthread,gandreello/openthread,JakubBrachTieto/openthread,mszczodrak/openthread,lanyuwen/openthread,aeliot/openthread,turon/openthread,aeliot/openthread,JakubBrachTieto/openthread,srickardti/openthread,JakubBrachTieto/openthread,pvanhorn/openthread