Add 3rd minimal whoosh index for performance

[UlrichB22]

Related to #1725.

After updating to this version the indexes need to be dropped and rebuild:

moin index-destroy
moin index-create
moin index-build

This will take some time for large wikis.

The index-build subcommand will create a new third index called LATEST_META. This index will be much smaller than LATEST_REVS and will only contain the usual metadata fields, but no content or content ngrams.

Many queries to check the existence of an item or to check ACL rights only require a few metadata fields. Queries against this small index are very fast and improve response times for large wikis.

The new index is called LATEST_META and the parameter to use the new index in various methods or functions is called “short”.

If you revert to a version prior to this change, you should delete and recreate the indexes. Advanced users can remove the new index files (latest_meta) in the wiki/index directory instead.

[RogerHaase]

A guess is you thought about removing content from the latest_revs indexes. Is it easier to create a 3rd index or did you do some benchmarking?

[UlrichB22]

A guess is you thought about removing content from the latest_revs indexes. Is it easier to create a 3rd index or did you do some benchmarking?

Some time ago I tested removing the content, this will impact searching and search results. IMO the different NGRAMs have the biggest impact on the index size. This solution with a third index should not change the search functionality.

[UlrichB22]

With the latest commit the recommended changes have been applied.
The new index is now called 'latest_meta'.

Running 'moin run' without recreating indexes will result in the following error message:

Error: Wiki index 'latest_meta' missing. Please see https://github.com/moinwiki/moin/pull/1877

[UlrichB22]

@ThomasWaldmann, @RogerHaase, may I ask you to review this PR. IMO this change is urgently needed for large wikis (e.g. python.org) to get reasonable response times. Thank you.

[RogerHaase]

Sorry for the delay. Busy with other things, no time for moin.

For my wiki on windows with 900+ items, response times dropped from about 5 seconds to about 2 seconds for the +index view. Very nice fix.