Optional Termlist Table

[olly]

The termlist table should be optional - without it, documents can't be deleted or replaced, and query expansion couldn't work, but most other things could be made to work.

Things which will now work:

• Database::alldocs_begin() - no longer uses the termlist.
• Database::get_doclength() - uses the document lengths stored in the postlist.

Things which currently use it:

• Determining the percentage scores when the top document doesn't match all the query terms.

Things which wouldn't work:

• WritableDatabase::delete_document()
• WritableDatabase::replace_document() if the document exists already.
• Database::termlist_begin()
• Document::termlist_begin()
• Document::termlist_count()
• Enquire::get_eset()

[olly]

No ABI changes, and existing flint database remain valid, so could be done in 1.0.x.

[richard]

I have no plans to work on this, so assigning to nobody.

[richard]

Currently, get_mset() uses the termlist for assigning percentages. This could be worked around by keeping track of which terms matched the current highest-relevance msetitem (actually, it would probably be easiest to track which Xapian::Weight::Internal objects were used for the best msetitem, since that will work for synonyms too). This information would need to be serialised along with the MSet, though, so will require a network protocol bump. We could probably manage to make this backwards compatible, so only a minor version bump, but it's easier to wait until we branch for 1.1.

[richard]

(In #120) Remove the unfixed dependencies so we can close this bug - they're all marked for the 1.1.0 milestone.

[richard]

Changed Component to Backend-Chert because flint is now "frozen", so we won't be working on this for flint.