irc: add server option "autojoin_delay" (closes #862)

The server option "autojoin_delay" adds a delay before autojoin.

The server option "command_delay" is now used to add a delay before the
execution of the command.

On upgrade from an old version, the option "command_delay" is copied to
"autojoin_delay" (in old versions, "command_delay" was applied after the
execution of command and before the autojoin).

tests/unit/plugins/irc/test-irc-server.cpp

@@ -1478,6 +1478,16 @@ TEST(IrcServer, DisconnectAll)
     /* TODO: write tests */
 }
 
+/*
+ * Tests functions:
+ *   irc_server_execute_command
+ */
+
+TEST(IrcServer, ExecuteCommand)
+{
+    /* TODO: write tests */
+}
+
 /*
  * Tests functions:
  *   irc_server_autojoin_create_buffers