1. fix issues on sending beacon caused by too much tx retries on other packets.
2. modify not to scan if rc exists when connect.
3. modify scan dwell time to default 120ms fo root.
1. fix a bug in initliazing map beacon interval.
2. fix not send MESH_EVENT_ROUTING_TABLE_REMOVE when MESH_NWK_CHILD_DISCONNECTED.
3. fix a bug in returning roots ie len.