refactor: Poll future, dropping task if entry is dropped or cancelled.

[dariusc93]

What this PR does 📖

• Poll future directly
• Drop task when cancelled or when entry is dropped
• Use rt::spawn instead for compatibility between native and wasm.

Which issue(s) this PR fixes 🔨

Relates to #494

Special notes for reviewers 🗒️

Additional comments 🎤