Serialize writes of JSON playlist

[darkdarkdragon]

What does this pull request do? Explain your changes. (required)
Fixes Error saving json playlist error.
We're making a lot of writes (to object store) to same json playlist file during stream.
Each write starts in own goroutine, so often new writes starts before previous completed.
In that case request to Google Cloud Storage often takes more than 10s, which is our timeout.
This PR fixes problem by serializing write requests.

Specific updates (required)

• Create new OvewriteQueue class that serializes writes to the same file in OS.
• Use OvewriteQueue for writing JSON playlists.

How did you test each of these updates (required)
Unit tests

Does this pull request close any open issues?
Fixes #1924

Checklist:

• Read the contribution guide
• make runs successfully
• All tests in ./test.sh pass
• README and other documentation updated
• Pending changelog updated

[jailuthra]

Before merge, would it be feasible to test it out in a local dev environment and/or staging, aside from the unit tests?

LGTM otherwise. Will let somebody else mark it as approved, as this is my first dive into this part of the codebase.

[yondonfu]

LGTM 🚢