Fit velo backward tracks with PrKalman

Fixing the fit of VELO backward tracks with PrKalmanFilter. Depends on !2722 (merged) and LHCb!3402 (merged).

For VELO backward tracks we now reverse the order of fit nodes and use a different downstream seed based on the most downstream fit node and slope of the CTB state.

Remaining inconsistency in resolution between TrackMasterFitter and PrKalmanFilter was solved by using TrackLinearExtrapolator for VELO tracks as TrackMasterFitter does by default.

@decianm @ausachov