fix(ext/http): Ensure cancelled requests don't crash Deno.serve (#19154)

Fixes for various `Attemped to access invalid request` bugs (#19058,
#15427, #17213).

We did not wait for both a drop event and a completion event before
removing items from the slab table. This ensures that we do so.

In addition, the slab methods are refactored out into `slab.rs` for
maintainability.

ext/http/lib.rs

@@ -81,6 +81,7 @@ mod reader_stream;
 mod request_body;
 mod request_properties;
 mod response_body;
+mod slab;
 mod websocket_upgrade;
 
 pub use request_properties::DefaultHttpPropertyExtractor;