kvserver: enqueue update check after Campaign()

I noticed that TestSplitBurstWithSlowFollower would average only
<10 splits per second even when no lagging replica is introduced (i.e.
the `time.Sleep` commented out). Investigating the raft chatter
suggested that after campaigning, the leaseholder of the right-hand
side would not be processed by its Store for a `Ready` handling cycle
until after ~50ms (the coalesced heartbeat timeout) in the test had
passed, and a similar delay was observed when it was sending out its
votes. Adding a call to `enqueueRaftUpdateCheck` fixes both, we end
up at ~100 splits per second.

Release note: None

pkg/kv/kvserver/replica_raft.go

@@ -1536,6 +1536,7 @@ func (r *Replica) maybeCampaignOnWakeLocked(ctx context.Context) {
  if err := r.mu.internalRaftGroup.Campaign(); err != nil {
  log.VEventf(ctx, 1, "failed to campaign: %s", err)
  }
+ r.store.enqueueRaftUpdateCheck(r.RangeID)
  }
 }