executor/tftpd: don't leak non-started connections

In the case a client would make a read request with options, and would fail to acknowledge the options reply, we would leak the connection forever.

Fix this by updating the sent_time property when sending the option reply, then kill the connection if the reply never got ACKed (leading to us trying to re-send a block we never sent to begin with).

Fixes: #172 (closed)