Use concurrent hashmap for headers in netty request

[justinlin-linkedin]

Summary

We are not ordering the headers and query parameters in netty request, just use HashMap, not TreeMap. And since we are now modifying the map from multiple threads due to 100-continue, we should use concurrenthashmap.

Test

Unit test

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 22.22222% with 7 lines in your changes missing coverage. Please review.

Project coverage is 70.10%. Comparing base (52ba813) to head (9064b07).
Report is 112 commits behind head on master.

Files with missing lines	Patch %	Lines
.../frontend/s3/S3MultipartCompleteUploadHandler.java	0.00%	1 Missing and 1 partial ⚠️
...mbry/frontend/s3/S3MultipartUploadPartHandler.java	0.00%	1 Missing and 1 partial ⚠️
...ava/com/github/ambry/frontend/s3/S3PutHandler.java	0.00%	1 Missing and 1 partial ⚠️
.../main/java/com/github/ambry/rest/NettyRequest.java	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2911      +/-   ##
============================================
+ Coverage     64.24%   70.10%   +5.85%     
- Complexity    10398    11908    +1510     
============================================
  Files           840      863      +23     
  Lines         71755    73375    +1620     
  Branches       8611     8838     +227     
============================================
+ Hits          46099    51438    +5339     
+ Misses        23004    19236    -3768     
- Partials       2652     2701      +49     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.