We already had haproxy configured to stop serving requests longer than
this duration, but the requests was still being processed by zope.

By adding such default in publisher-timeout, zope will stop processing
the request after the timeout.

This is slightly different behavior, because before this change the
request was still being processed, even if the client did not get the
response it may make change to databases.
Users who want to keep the previous behavior can request with
publisher-timeout parameter null.