add mutex to read/write http headers

[pawelsz-rb]

Description of the change

Adding a proper mutex to read/write http headers.

Note for the reviewer: I tried to optimize the code by locking/unlocking the mutex explicitly exactly when it was needed (overall execution little bit faster), instead of deferring unlocking, which would make the code more elegant and easier to read. (overall execution little bit slower). Please let me know which way you would prefer.

Type of change

• [x] Bug fix (non-breaking change that fixes an issue)
• [ ] New feature (non-breaking change that adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Maintenance
• [ ] New release

Related issues

Shortcut stories and GitHub issues (delete irrelevant)

• Fix #346

Checklists

Development

• [ ] Lint rules pass locally
• [ ] The code changed/added as part of this pull request has been covered with tests
• [ ] All tests related to the changed code pass in development

Code review

• [ ] This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
• [ ] "Ready for review" label attached to the PR and reviewers assigned
• [ ] Issue from task tracker has a link to this pull request
• [ ] Changes have been reviewed by at least one other engineer

[github-actions[bot]]

:tada: This PR is included in version 1.11.0 :tada:

The release is available on:

• GitHub release
• v1.11.0

Your semantic-release bot :package::rocket: