Optimize DLedgerLeaderElector.maintainAsCandidate code

[mxsm]

As following picture shows:

            if (!memberState.isCandidate()) {
                return;
            }

This code can also be added before synchronized, Double check reduces locking. code as following

        long term;
        long ledgerEndTerm;
        long ledgerEndIndex;
        if (!memberState.isCandidate()) {
            return;
        }
        synchronized (memberState) {
            if (!memberState.isCandidate()) {
                return;
            }
            if (lastParseResult == VoteResponse.ParseResult.WAIT_TO_VOTE_NEXT || needIncreaseTermImmediately) {
                long prevTerm = memberState.currTerm();
                term = memberState.nextTerm();
                logger.info("{}_[INCREASE_TERM] from {} to {}", memberState.getSelfId(), prevTerm, term);
                lastParseResult = VoteResponse.ParseResult.WAIT_TO_REVOTE;
            } else {
                term = memberState.currTerm();
            }
            ledgerEndIndex = memberState.getLedgerEndIndex();
            ledgerEndTerm = memberState.getLedgerEndTerm();
        }

and switch case

                    synchronized (knownMaxTermInGroup) {
                        switch (x.getVoteResult()) {
                            case ACCEPT:
                                acceptedNum.incrementAndGet();
                                break;
                            case REJECT_ALREADY_VOTED:
                            case REJECT_TAKING_LEADERSHIP:
                                break;
                            case REJECT_ALREADY_HAS_LEADER:
                                alreadyHasLeader.compareAndSet(false, true);
                                break;
                            case REJECT_TERM_SMALL_THAN_LEDGER:
                            case REJECT_EXPIRED_VOTE_TERM:
                                if (x.getTerm() > knownMaxTermInGroup.get()) {
                                    knownMaxTermInGroup.set(x.getTerm());
                                }
                                break;
                            case REJECT_EXPIRED_LEDGER_TERM:
                            case REJECT_SMALL_LEDGER_END_INDEX:
                                biggerLedgerNum.incrementAndGet();
                                break;
                            case REJECT_TERM_NOT_READY:
                                notReadyTermNum.incrementAndGet();
                                break;
                            default:
                                break;

                        }
                    }

REJECT_ALREADY_VOTED、REJECT_TAKING_LEADERSHIP branchs in 'switch' is a duplicate of the default branch, we can merge like this

                    synchronized (knownMaxTermInGroup) {
                        switch (x.getVoteResult()) {
                            case ACCEPT:
                                acceptedNum.incrementAndGet();
                                break;
                            case REJECT_ALREADY_HAS_LEADER:
                                alreadyHasLeader.compareAndSet(false, true);
                                break;
                            case REJECT_TERM_SMALL_THAN_LEDGER:
                            case REJECT_EXPIRED_VOTE_TERM:
                                if (x.getTerm() > knownMaxTermInGroup.get()) {
                                    knownMaxTermInGroup.set(x.getTerm());
                                }
                                break;
                            case REJECT_EXPIRED_LEDGER_TERM:
                            case REJECT_SMALL_LEDGER_END_INDEX:
                                biggerLedgerNum.incrementAndGet();
                                break;
                            case REJECT_TERM_NOT_READY:
                                notReadyTermNum.incrementAndGet();
                                break;
                            case REJECT_ALREADY_VOTED:
                            case REJECT_TAKING_LEADERSHIP:
                            default:
                                break;

                        }
                    }