use stricter mutex policy to prevent installed.json corruption

[umbynos]

Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• Tests for the changes have been added (for bug fixes / features)

• What kind of change does this PR introduce?

Bug fix, followup of #585

• What is the current behavior?

Sometimes the agent responds with a 500 error to curl ‘http://127.0.0.1:8991/v2/pkgs/tools/installed’ with this payload {“name”:“fault”,“id”:“JOhq4y8g”,“message”:“invalid character ‘s’ after top-level value”,“temporary”:false,“timeout”:false,“fault”:true}.
This error should be caused by a race condition with the result that installed.json file gets corrupted

• What is the new behavior?

This PR should fix this behavior by adding a stricter mutex policy and releasing the mutex only after the file has been written

• Does this PR introduce a breaking change?

no

• Other information:

[umbynos]

To test this change I created 1.2.3-rc1