relay: fix crash on /upgrade received from a client (weechat protocol)

Some commands like /upgrade sent by relay client can cause problems, because
they were executed immediately (while relay code is running). The /upgrade
command unloads all plugins, so the result is unpredictable (it can cause a
crash).

This commit adds a timer (1 millisecond) to delay the execution of command
after we go back in the WeeChat main loop.