async: Increase file descriptor limit for async

The file descriptor limit seems to be set to 1024 by async. This commit
makes katnetic use a different entry point for the scheduler, which
allows the user to configure the maximum number of file descriptors.

Related to frenetic-lang/ocaml-openflow#70.

frenetic/katnetic.ml

@@ -16,9 +16,11 @@ module Run = struct
   open LocalCompiler.RunTime
 
   let with_channel f chan =
+    let open Core.Std in
+    let open Async.Std in
     let exp = Parser.program Lexer.token (Lexing.from_channel chan) in
-    let _ = Async_Controller.start_static f 6633 exp in
-    Core.Std.never_returns (Async.Std.Scheduler.go ())
+    let main () = ignore (Async_Controller.start_static f 6633 exp) in
+    never_returns (Scheduler.go_main ~max_num_open_file_descrs:4096 ~main ())
 
   let with_file f filename =
     with_channel f (open_in filename)