compio: work still queued when a runtime drops fails compio-executor's drop assertion #156

After moving to http-kit-io 0.6, three of zenwave's compio tests on Linux panic when their runtime drops: h3_after_alt_svc, h3_delivers_response_trailers and h3_black_hole_falls_back. The panic is assertion failed: inner.map.is_empty() at compio-executor 0.1.4 src/queue.rs:104. The same tests pass on macOS and Windows with compio and on Linux with tokio, and passed against an earlier http-kit main. The Linux-only work that landed in between spawns background tasks: the file-slot install and release (#153) and the zero-copy notification holder (#152).

Done when dropping a compio runtime with that work outstanding no longer trips the assertion, a regression test in http-kit covers it, and slots are still never leaked and zero-copy buffers never freed before their notification.