Browse middleware is synchronous and slow

[danneu]

The handler calls synchronous file-system APIs on the main thread like File.open, fs::read_dir (which incurs opendir once and then readdir on each next() of the iterator).

Each service's call() should do what Root's does and dispatch Req->Res on the thread pool. But it's not clear to me if this is the only problem.

Serve a directory with 5000 entries in it (mkdir dir_{1..5000}). Maxes out at 12 req/sec on my machine.