feat: add starting point

src/main/java/com/limechain/grandpa/round/GrandpaRound.java

@@ -245,7 +245,7 @@ private boolean isFinalizable() {
      *
      * @return if the current round is completable
      */
-    private boolean isCompletable() {
+    public boolean isCompletable() {
 
         Map<Vote, Long> votes = getDirectVotes(SubRound.PRE_COMMIT);
         long votesCount = votes.values().stream()

src/main/java/com/limechain/grandpa/round/PreVoteStage.java

@@ -1,13 +1,56 @@
 package com.limechain.grandpa.round;
 
+import lombok.extern.java.Log;
+
+import java.util.concurrent.TimeUnit;
+
+@Log
 public class PreVoteStage implements StageState {
 
     @Override
     public void start(GrandpaRound round) {
+        if (round.isCompletable()) {
+            log.fine(String.format("Round %d is completable.", round.getRoundNumber()));
+            end(round);
+            return;
+        }
+
+        long duration = 0; //TODO: choose appropriate value
+        long delay = (duration * 2) - (System.currentTimeMillis() - round.getStartTime().toEpochMilli());
+
+        round.getOnStageTimerHandler().schedule(() -> {
+            log.info(String.format("Round #%d: Time of prevote stage is out", round.getRoundNumber()));
+            end(round);
+        }, delay, TimeUnit.MILLISECONDS);
+
     }
 
     @Override
     public void end(GrandpaRound round) {
+        log.info(String.format("Round %d ended pre-vote stage", round.getRoundNumber()));
+        round.setState(new PreCommitStage());
     }
+
+//    stage_timer_handle_ = scheduler_->scheduleWithHandle(
+//        [wself{weak_from_this()}] {
+//        if (auto self = wself.lock()) {
+//            if (self->stage_ == Stage::PREVOTE_RUNS) {
+//                SL_DEBUG(self->logger_,
+//                        "Round #{}: Time of prevote stage is out",
+//                        self->round_number_);
+//                self->endPrevoteStage();
+//            }
+//        }
+//    },
+//    toMilliseconds(duration_ * 2 - (scheduler_->now() - start_time_)));
+//
+//    on_complete_handler_ = [this] {
+//        if (stage_ == Stage::PREVOTE_RUNS) {
+//            SL_DEBUG(logger_, "Round #{}: Became completable", round_number_);
+//            endPrevoteStage();
+//        }
+//    };
+//
+//    stage_ = Stage::PREVOTE_RUNS;
 }