fts xapian does not publish configuration changes in a changelog. As a
result, some options that nixos mailserver was setting for it have been
ignored for several years. New options (process_limit) are now
recommended. This adapts the module to these changes.
The default value of partial= is 2, but fts_xapian 1.8.3 now requires it
to be at least 3, and fails loudly in case it is 2. As a result, this
change is required to support fts_xapian 1.8.3 and later.