proxy: cull http connections (#7632)

## Problem

Some HTTP client connections can stay open for quite a long time.

## Summary of changes

When there are too many HTTP client connections, pick a random
connection and gracefully cancel it.

proxy/Cargo.toml

@@ -40,6 +40,7 @@ hyper.workspace = true
 hyper1 = { package = "hyper", version = "1.2", features = ["server"] }
 hyper-util = { version = "0.1", features = ["server", "http1", "http2", "tokio"] }
 http-body-util = { version = "0.1" }
+indexmap.workspace = true
 ipnet.workspace = true
 itertools.workspace = true
 lasso = { workspace = true, features = ["multi-threaded"] }