[SCB-1528] Optimizing some code implementations

[AngLi2]

Follow this checklist to help us incorporate your contribution quickly and easily:

• Make sure there is a JIRA issue filed for the change (usually before you start working on it). Trivial changes like typos do not require a JIRA issue. Your pull request should address just this issue, without pulling in other changes.
• Each commit in the pull request should have a meaningful subject line and body.
• Format the pull request title like [SCB-XXX] Fixes bug in ApproximateQuantiles, where you replace SCB-XXX with the appropriate JIRA issue.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Run mvn clean install -Pit to make sure basic checks pass. A more thorough check will be performed on your pull request automatically.
• If this contribution is large, please file an Apache Individual Contributor License Agreement.

---

[AngLi2]

1. Iterating entrySet() is more efficent than entryKey() when call map.get(key) each time;
2. Collections.isEmpty() is more efficent than collections.size() == 0
3. Util classes should have private constructures
4. String.valueOf() is more efficent then + ""
5. BigDecimal() may cause some precision problems while BigDecimal.valueOf() not
6. Return empty collection instead of null can prevent NPE
7. Objects.equals(a, b) is better than a.equals(b) because of NPE
8. Data in enum shoud be private final

[liubao68]

This test case is useful that we must gurrantee we can call getOrCreateMicroserviceVersionRule after BEFORE_REGISTRY without exception throw in bootup.

[coveralls]

Coverage decreased (-0.006%) to 85.874% when pulling 90299b2 on AngLi2:optimize into 4595644 on apache:master.

[liubao68]

It seems this code does not use servicecomb recommented formatter.