Improve the speed of FAISSDocumentStore.delete_documents()

[akkefa]

fixed #647

Database Delete benchmark

Current delete query took 12 m 54 s 892 ms on 10000000 records

DELETE FROM document WHERE document.id IN (SELECT document.id
                                                           FROM document
                                                           WHERE document.index = 'doc')

Updated delete query took 1 m 26 s 147 ms on 10000000 records

DELETE FROM document WHERE document.index = 'doc'

Sample data generation query:

insert into document ( id, text, index ) select s, md5(random()::text), 'doc' from generate_series(1, 10000000) s(i)

Status (please check what you already did):

• First draft (up for discussions & feedback)
• Final code
• [] Added tests
• Updated documentation

[akkefa]

@tholor Please review this pull request.

[tholor]

LGTM! Thanks for the fix!