discover: Don't free clients on write failure

discover: Don't free clients on write failure

Currently, if we see an error when writing to a client, we free the
client immediately. This means that any pending reads will be discarded.
Additionally, if a client disconnects before we have finished the writes in
discover_server_process_connection, we won't register the waiter to read
from the socket.

Instead of freeing the client on write failure, this change sets a flag
(se we don't continue to write), and we rely on the read EOF to free the
client. This means we don't drop incoming messages.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>