improve database performance and reliability

Summary:
let database test addOneTrackWithoutAlbumArtist have a file database

let the numeric unique id starts at 1 instead of 2

let fix warnings from QtCreator in database tests

DatabaseInterface: does not emit track changed signals for newly added

album artist name is directly a foreign key in Albums

should help to ensure greater reliability

lists of authors where not working so let's scrap this

use the unique fields of Albums for foreign key in Tracks

remove TracksArtists Table

no longer store tracks count and single disc states in albums table

reduce overhead from AllTracksModel

reduce overhead of AllAlbumsModel and make it synchronous

do not list files too early from Baloo

keep all files discovered from a source and fix a minor related issue

add a test for DatabaseInterface::askRestoredTracks

add albums and artists cache and improve cache usage

improve management of files not modified since last scan of music files

CCBUG: 389136
CCBUG: 396607
CCBUG: 396840

Test Plan: tests are OK and Elisa application is still working

Reviewers: astippich, januz

Reviewed By: astippich

Tags: Elisa

Differential Revision: https://phabricator.kde.org/D16302