locust: Use a new session for each request

[akashin]

This allows parallelism across requests issued through the same NearNodeProxy. Alternative would be to use different proxies, but this can be tricky in calls like on_locust_init and ft_contract.create_passive_users.

[codecov[bot]]

Codecov Report

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

Project coverage is 71.47%. Comparing base (3e3137f) to head (12fd557). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #11622 +/- ## ========================================== + Coverage 71.46% 71.47% +0.01% ========================================== Files 788 788 Lines 160854 160854 Branches 160854 160854 ========================================== + Hits 114947 114964 +17 + Misses 40887 40869 -18 - Partials 5020 5021 +1 ``` | [Flag](https://app.codecov.io/gh/near/nearcore/pull/11622/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | Coverage Δ | | |---|---|---| | [backward-compatibility](https://app.codecov.io/gh/near/nearcore/pull/11622/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `0.23% <ø> (ø)` | | | [db-migration](https://app.codecov.io/gh/near/nearcore/pull/11622/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `0.23% <ø> (ø)` | | | [genesis-check](https://app.codecov.io/gh/near/nearcore/pull/11622/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `1.35% <ø> (ø)` | | | [integration-tests](https://app.codecov.io/gh/near/nearcore/pull/11622/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `37.69% <ø> (-0.01%)` | :arrow_down: | | [linux](https://app.codecov.io/gh/near/nearcore/pull/11622/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `68.88% <ø> (-0.01%)` | :arrow_down: | | [linux-nightly](https://app.codecov.io/gh/near/nearcore/pull/11622/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `70.96% <ø> (+0.02%)` | :arrow_up: | | [macos](https://app.codecov.io/gh/near/nearcore/pull/11622/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `50.96% <ø> (-1.60%)` | :arrow_down: | | [pytests](https://app.codecov.io/gh/near/nearcore/pull/11622/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `1.59% <ø> (ø)` | | | [sanity-checks](https://app.codecov.io/gh/near/nearcore/pull/11622/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `1.38% <ø> (ø)` | | | [unittests](https://app.codecov.io/gh/near/nearcore/pull/11622/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `66.13% <ø> (+<0.01%)` | :arrow_up: | | [upgradability](https://app.codecov.io/gh/near/nearcore/pull/11622/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `0.28% <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=near#carryforward-flags-in-the-pull-request-comment) to find out more.

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