Improve shutdown of tasks

[phughk]

Thank you for submitting this pull request! We really appreciate you spending the time to work on these changes.

What is the motivation?

Flume channel into_stream does not terminate if a channel is dropped.
https://docs.rs/flume/latest/flume/struct.Receiver.html#method.stream

What does this change do?

Fix termination

What is your testing strategy?

RUST_LOG=trace cargo test --package surrealdb --no-default-features --features kv-mem api::engine::tasks::test::tasks_complete_channel_closed -- --nocapture

Is this related to any issues?

Potentially tied to #3906 , but not the main cause

Does this change need documentation?

• No documentation needed
• Feat(docusaurus): Adds styling, theming and menus docs.surrealdb.com#1

Have you read the Contributing Guidelines?

• I have read the Contributing Guidelines