Flush spool queue for console terminate as well

Mail spool were only flush at the end of a HTTP Request (KernelEvents:TERMINATE). In case of a console command sending mail using a spool, the spool was not flushed, so no mail was sent (and in case of MemorySpool, the mail were lost).

So this PR subscribes to the ConsoleEvents:TERMINATE to do the same flushing as in the context of a HTTP request.