fix: OPTIC-1373: Update async import and reimport to be on by default and amend locust tests

[bmartel]

PR fulfills these requirements

• [X] Commit message(s) and PR title follows the format [fix|feat|ci|chore|doc]: TICKET-ID: Short description of change made ex. fix: DEV-XXXX: Removed inconsistent code usage causing intermittent errors
• [X] Tests for the changes have been added/updated (for bug fixes/features)
• [ ] Docs have been added/updated (for bug fixes/features)
• [X] Best efforts were made to ensure docs/code are concise and coherent (checked for spelling/grammatical errors, commented out code, debug logs etc.)
• [X] Self-reviewed and ran all changes on a local instance (for bug fixes/features)

Change has impacts in these area(s)

(check all that apply)

• [ ] Product design
• [ ] Backend (Database)
• [X] Backend (API)
• [ ] Frontend

Describe the reason for change

Locust tests failed to run with async import and reimport on, staling the FFs involved and updating the tests to be compatible.

What feature flags were used in this change?

• fflag_feat_all_lsdv_4915_async_task_import_13042023_short
• fflag_fix_all_lsdv_4971_async_reimport_09052023_short

Does this PR introduce a breaking change?

(check only one)

• [ ] Yes, and covered entirely by feature flag(s)
• [ ] Yes, and covered partially by feature flag(s)
• [X] No
• [ ] Not sure (briefly explain the situation below)

What level of testing was included in the change?

(check all that apply)

• [ ] e2e
• [X] integration
• [ ] unit

Which logical domain(s) does this change affect?

Async Import/Reimport

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 76.77%. Comparing base (86c907c) to head (fe490a8). Report is 5 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #6696 +/- ## ======================================== Coverage 76.77% 76.77% ======================================== Files 170 170 Lines 13883 13883 ======================================== Hits 10659 10659 Misses 3224 3224 ``` | [Flag](https://app.codecov.io/gh/HumanSignal/label-studio/pull/6696/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=HumanSignal) | Coverage Δ | | |---|---|---| | [pytests](https://app.codecov.io/gh/HumanSignal/label-studio/pull/6696/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=HumanSignal) | `76.77% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=HumanSignal#carryforward-flags-in-the-pull-request-comment) to find out more.

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