Only set RequestCtx.s once

We have a RequestCtx pool per server so we only need to set it once.
This fixes a race codition where acquireCtx would assign .s while
.Done() is reading it. Even though acquireCtx would always set it to the
same, already assigned, value it would still trigger the race detector.

Fixes: #1261

server.go

@@ -2634,11 +2634,11 @@ func (s *Server) acquireCtx(c net.Conn) (ctx *RequestCtx) {
 		ctx = new(RequestCtx)
 		ctx.Request.keepBodyBuffer = keepBodyBuffer
 		ctx.Response.keepBodyBuffer = keepBodyBuffer
+		ctx.s = s
 	} else {
 		ctx = v.(*RequestCtx)
 	}
 
-	ctx.s = s
 	ctx.c = c
 
 	return ctx