journal-remote: increase memory limit per connection to avoid errors with HTTPS

journal-remote: increase memory limit per connection to avoid errors with HTTPS

Explicitly set MHD_OPTION_CONNECTION_MEMORY_LIMIT to a larger value,
when setting up microhttpd, to give more memory per HTTP(S) connection.

This way systemd-journal-remote can now prevent microhttpd from failing
in creating response headers with messages like "Not enough memory for
write", especially when lots of HTTPS requests arrive. That's precisely
because MHD_OPTION_CONNECTION_MEMORY_LIMIT in libmicrohttpd defaults to
32768, which is in practice insufficient in this case.
See also https://gnunet.org/bugs/view.php?id=4007 for more details.

Fixes: https://github.com/coreos/bugs/issues/927