[Bugfix] Fix NPE caused by concurrent access

[banmoy]

What type of PR is this：

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Which issues of this PR fixes ：

Fixes #

Problem Summary(Required) ：

java.lang.NullPointerException: null;
at java.util.LinkedList$ListItr.next(LinkedList.java:893);
at com.starrocks.connector.flink.manager.StarRocksSinkManagerV2.lambda$init$0(StarRocksSinkManagerV2.java:158);

StreamLoadManagerV2#flushQ could be read and write concurrently, but it's a LinkedList which is not a concurrent data structure. We can replace it with ConcurrentLinkedQueue. At the same time, fix the potential concurrent problem for regions.

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr will affect users' behaviors
• This pr needs user documentation (for new or modified features or behaviors)
• I have added documentation for my new feature or new function