Remove Buf_io from the HTTP server (#6064)

Cleaning up the attic... It turned out that the `Buf_io` module, while
used in the type signature of every HTTP handler, was not used for
anything useful anymore.

Step by step rationale in the commit messages, so best reviewed commit
by commit.

ocaml/database/database_server_main.ml

@@ -80,9 +80,9 @@ let _ =
         let socket = Http_svr.bind sockaddr "unix_rpc" in
         let server = Http_svr.Server.empty () in
         Http_svr.Server.add_handler server Http.Post "/post_remote_db_access"
-          (Http_svr.BufIO remote_database_access_handler_v1) ;
+          remote_database_access_handler_v1 ;
         Http_svr.Server.add_handler server Http.Post "/post_remote_db_access_v2"
-          (Http_svr.BufIO remote_database_access_handler_v2) ;
+          remote_database_access_handler_v2 ;
         Http_svr.start ~conn_limit:1024 server socket ;
         Printf.printf "server listening\n%!" ;
         if !self_test then (

ocaml/database/db_remote_cache_access_v1.ml

@@ -126,11 +126,10 @@ module DBCacheRemoteListener = struct
         raise e
 end
 
-let handler req bio _ =
-  let fd = Buf_io.fd_of bio in
+let handler req fd _ =
   (* fd only used for writing *)
   let body =
-    Http_svr.read_body ~limit:Db_globs.http_limit_max_rpc_size req bio
+    Http_svr.read_body ~limit:Db_globs.http_limit_max_rpc_size req fd
   in
   let body_xml = Xml.parse_string body in
   let reply_xml = DBCacheRemoteListener.process_xmlrpc body_xml in

ocaml/database/db_remote_cache_access_v1.mli

@@ -1,2 +1,2 @@
-val handler : Http.Request.t -> Buf_io.t -> 'a -> unit
+val handler : Http.Request.t -> Unix.file_descr -> 'a -> unit
 (** HTTP handler for v1 of the remote DB access protocol *)

ocaml/database/db_remote_cache_access_v2.ml

@@ -67,11 +67,10 @@ let process_rpc (req : Rpc.t) =
         Response.Too_many_values (x, y, z)
     )
 
-let handler req bio _ =
-  let fd = Buf_io.fd_of bio in
+let handler req fd _ =
   (* fd only used for writing *)
   let body =
-    Http_svr.read_body ~limit:Db_globs.http_limit_max_rpc_size req bio
+    Http_svr.read_body ~limit:Db_globs.http_limit_max_rpc_size req fd
   in
   let request_rpc = Jsonrpc.of_string body in
   let reply_rpc = process_rpc request_rpc in

ocaml/database/db_remote_cache_access_v2.mli

@@ -1,2 +1,2 @@
-val handler : Http.Request.t -> Buf_io.t -> 'a -> unit
+val handler : Http.Request.t -> Unix.file_descr -> 'a -> unit
 (** HTTP handler for v2 of the remote DB access protocol *)