Replace `JsInstance` pool with single worker and FIFO queue #20416

Job	Run time
Check CI command docs	23s
Test spacetimedb-update flow (x86_64-pc-windows-msvc)	26s
Build and test wasm bindings	24s
Test spacetimedb-update flow (aarch64-apple-darwin)	28s
Check smoketests/mod.rs is complete	21s
Check CLI docs	23s
Check for Python smoketest edits	14s
Test spacetimedb-update flow (x86_64-unknown-linux-gnu)	24s
Verify global.json files are symlinks	21s
Lints	24s
Test spacetimedb-update flow (aarch64-unknown-linux-gnu)	37s
Check that packages are publishable	27s
Verify LLM benchmark is up to date	0s
Unreal Engine Tests	0s
csharp-testsuite	1s
Smoketests (${{ matrix.name }})	1s
Test Suite	1s
Internal Tests	1s
unity-testsuite	1s
	4m 57s

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