http: replace io with fio module

In tarantool the standart built-in module for working with files is "fio".
So we should use it instead of "io".

Patch adds a commit 'Get rid of io module'
(e0d8f82) that was implemented for http
v2 and later reverted in scope of issue with discard v2.

Follows up #90
Part of #134

http/server.lua

@@ -2,7 +2,7 @@
 
 local lib = require('http.lib')
 
-local io = io
+local fio = require('fio')
 local require = require
 local package = package
 local mime_types = require('http.mime_types')
@@ -349,8 +349,17 @@ local function load_template(self, r, format)
 
 
     local tpl = catfile(self.options.app_dir, 'templates', file)
-    local fh = io.input(tpl)
-    local template = fh:read('*a')
+    local fh, err = fio.open(tpl)
+    if err ~= nil then
+        errorf("Can not load template for '%s': '%s'", r.path, err)
+    end
+
+    local template
+    template, err = fh:read()
+    if err ~= nil then
+        errorf("Can not load template for '%s': '%s'", r.path, err)
+    end
+
     fh:close()
 
     if self.options.cache_templates then
@@ -540,14 +549,18 @@ local function static_file(self, request, format)
             }
         end
 
-        local s, fh = pcall(io.input, file)
+	local fh, err = fio.open(file, {'O_RDONLY'})
+	if err ~= nil then
+	    return { status = 404 }
+	end
 
-        if not s then
-            return { status = 404 }
-        end
+	local body
+	body, err = fh:read()
+	if err ~= nil then
+	    errorf("Can not return static file for '%s': '%s'", request:path(), err)
+	end
 
-        local body = fh:read('*a')
-        io.close(fh)
+	fh:close()
 
         if self.options.cache_static then
             self.cache.static[ file ] = body