Assertion failed: Write.cc:38: "fd_table[conn->fd].flags.open"

Assertion failed: Write.cc:38: "fd_table[conn->fd].flags.open"

The Ftp::Server::stopWaitingForOrigin() notification may come after
Ftp::Server (or an external force) has started closing the control
connection but before the Ftp::Server job became unreachable for
notifications. Writing a response in this state leads to assertions.

Other, currently unknown paths may lead to the same write-after-close
problems. This change protects all asynchronous notification methods
(except the connection closure handler itself) from exposing underlying
code to a closing control connection. This is very similar to checking
for ERR_CLOSING in Comm handlers.

This is a Measurement Factory project.