Replace synchronized block with Lock

[TharmiganK]

Purpose

$Subject

Completes: ballerina-platform/ballerina-library#7488

Tests

Checked with the following number of users with 50B payload. Worked without any hanging issues

• 200

• 300

• 500

• 600

Checklist

• Linked to an issue
• Updated the changelog
• Added tests
• Updated the spec
• Checked native-image compatibility
• Checked the impact on OpenAPI generation

[codecov]

Codecov Report

Attention: Patch coverage is 96.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.83%. Comparing base (b58e52d) to head (28745f6).

Files with missing lines	Patch %	Lines
...rt/contractimpl/sender/http2/Http2ChannelPool.java	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2265      +/-   ##
============================================
- Coverage     80.48%   79.83%   -0.65%     
- Complexity        0      678     +678     
============================================
  Files           366      418      +52     
  Lines         20749    23122    +2373     
  Branches       3289     3750     +461     
============================================
+ Hits          16700    18460    +1760     
- Misses         3119     3526     +407     
- Partials        930     1136     +206     

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud