fix httphandler async query.

[youngsofun]

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Summary

request blocked because the interpreter is called in handler task. move it to the spawned task.
it works for select which get a stream immediately and the caller wait on it.
but errors show up for queries like COPY, which return a useless stream after real work is done. cc @ZhiHanZ

Changelog

Related Issues

Fix #4297

Test Plan

Unit Tests

Stateless Tests

long copy tested on my mac

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[mergify]

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[Xuanwo]

Convert to ready for review so that this PR can get merged.

[youngsofun]

@Xuanwo thank you, some tests failed in CI (succeed on my mac), still debugging