allow to specify how many times we can try to connect to the database at startup

add config options:
db_startup_connection_max_attempts - default 0 (infinite)
db_startup_connection_attempt_delay - delay in seconds between attempts (default 5)

BREAKING CHANGE: WINIX_PLUGIN_INIT plugin message requires to set result status,
you have to set the result status to true (env.res) if your
plugin was initialized correctly, otherwise winix will not start

winixd/core/config.h

@@ -242,12 +242,21 @@ public:
 	std::wstring db_user;
 	std::wstring db_pass;
 
+	// specify how many times we can try to connect to the database at startup
+	// default 0 (infinite)
+	size_t db_startup_connection_max_attempts;
+
+	// delay between each connection attempt at startup
+	// default 5 (seconds)
+	size_t db_startup_connection_attempt_delay;
+
 	// make database migration if needed
-	//
 	bool db_make_migration_if_needed;
 
+	// if a migration fails then stop winix
 	bool db_stop_if_migration_fails;
 
+
 	// the name of the cookie which has the session identifier
 	std::wstring http_session_id_name;