Speed up asynchttpserver significantly using all the previous changes

- Export socket field of AsyncHttpServer and addHeaders proc for templates

- Make respond a template instead of proc because of how often it's called.
  This means no more "await" when invoking it.

- Optimize respond template with special case for empty headers and
  Content-Length entry

- newRequest doesn't allocate a hostname and body anymore because they're
  copied in later

- Major changes to processClient to prevent allocations and copies

lib/pure/asyncnet.nim

@@ -200,7 +200,7 @@ template readInto*(buf: cstring, size: int, socket: AsyncSocket,
     res = recvIntoFut.read()
   res
 
-template readIntoBuf(socket: AsyncSocket,
+template readIntoBuf*(socket: AsyncSocket,
     flags: set[SocketFlag]): int =
   var size = readInto(addr socket.buffer[0], BufferSize, socket, flags)
   socket.currPos = 0