Mustache template dir is set statically but in each request

[gittiver]

On review of
#1187

I noticed that the mustache base dir is set statically. As request are running in parallel this could lead to using a wrong template base dir
for the template in the request.

This behavior is a bug in my opinion which should be further investigated and fixed.

[jwolak]

Hello,

I'd like to propose a fix:

--- a/include/crow/mustache.h
+++ b/include/crow/mustache.h
@@ -745,16 +745,15 @@ namespace crow // NOTE: Already documented in "crow/app.h"
 
         namespace detail
         {
-            inline std::string& get_template_base_directory_ref()
+            inline std::string& get_global_template_base_directory_ref()
             {
                 static std::string template_base_directory = "templates";
                 return template_base_directory;
             }
 
-            /// A base directory not related to any blueprint
-            inline std::string& get_global_template_base_directory_ref()
+            inline std::string& get_template_base_directory_ref()
             {
-                static std::string template_base_directory = "templates";
+                thread_local std::string template_base_directory = get_global_template_base_directory_ref();
                 return template_base_directory;
             }
         } // namespace detail