mrp: changes to LVA_TIMER event handling

1) mrp_lvatimer_fsm() explicitly returns tx_request (LVA_TIMER event is the only evt that cares).
2) Add MRP_EVENT_SLA. sLA processing the same as rLA with the exception that mrp_lvatimer_fsm() is not run.
   If mrp_lvatimer_fsm() was run, the lvatimer S/M would transition to PASSIVE and a tx! would then fail
   to set lva.tx.
3) When mrp_lvatimer_fsm() returns tx_request, an sLA event is generated.
4) lva.sdnmsg was never used, so don't set it in mrp_lvatimer_fsm()