[mpl] fix inserting mechanism and timer handling (#3587)
[mpl] fix inserting mechanism and timer handling (#3587)


chshu/openthread,librasungirl/openthread,abtink/openthread,jwhui/openthread,srickardti/openthread,turon/openthread,lanyuwen/openthread,abtink/openthread,chshu/openthread,turon/openthread,abtink/openthread,openthread/openthread,bukepo/openthread,srickardti/openthread,lanyuwen/openthread,srickardti/openthread,bukepo/openthread,librasungirl/openthread,chshu/openthread,lanyuwen/openthread,lanyuwen/openthread,abtink/openthread,openthread/openthread,lanyuwen/openthread,jwhui/openthread,turon/openthread,jwhui/openthread,chshu/openthread,lanyuwen/openthread,openthread/openthread,bukepo/openthread,jwhui/openthread,chshu/openthread,librasungirl/openthread,chshu/openthread,openthread/openthread,turon/openthread,librasungirl/openthread,bukepo/openthread,srickardti/openthread