*: remove unnecessary async blocks to save memory (#16541)

[ti-chi-bot]

This is an automated cherry-pick of #16541

What is changed and how it works?

Issue Number: Close #16540

Cc #16234

What's Changed:

*: enable linters about async and futures
  We should be pedantic about writing async code, as it's easy to write
  suboptimal or even bloat code.
  See: https://github.com/rust-lang/rust/issues/69826

*: remove unnecessary async blocks to save memory
  This commit favors FutureExt::map over async blocks to mitigate
  the issue of async block doubled memory usage. Through the sysbench
  oltp_read_only test, it was observed that this adjustment resulted
  in approximately 26% reduction in memory usage.
  See: https://github.com/rust-lang/rust/issues/59087

Check List

Tests

• Unit test
• Manual test

Run sysbench oltp_read_only on a single-node tikv cluster.

Configs and test commands

TiKV config:

[readpool]
[readpool.unified]
# Unlimited concurrent running tasks.
max-tasks-per-worker = 1000000000

Injects a fail points to TiKV so that all tasks on read pool are blocked.

TiDB config:

# Do not limit concurrent transcations.
token-limit = 100000000
[txn-local-latches]
enabled = false

Sysbench command:

while true; do { \
    sysbench --db-driver=mysql --mysql-host=10.2.12.86 --mysql-port=31825 --mysql-user=root \
        --mysql-db=test1 --tables=1 --table-size=100000000 --create_secondary=false \
        --time=5 --skip_trx=true --range_selects=false  --threads=1000 ./oltp_read_only run & \
    pid=$!; sleep 8; kill $pid; \
} done

Test results:

Nightly vs This PR 4.47GB vs 3.28GB

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Connor1996
• overvenus

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[Connor1996]

LGTM

[overvenus]

/merge

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 03dd055

[ti-chi-bot]

@ti-chi-bot: Your PR was out of date, I have automatically updated it for you.

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