session pairing: simplify and move session pairing.
session pairing doesn't compute anymore the shape of the goal, it is done before. It was able to compute the shape only when the checksum of the task was different, but computing the checksum of the task is way more time consuming than computing the shape of the goal (and include it). So this commit simplify greatly the function and theoretically augment just a little the time spent. Experimentaly it's the inverse on max_matrix. Until "update_session: done" with or without modifying the checksums: before | after without : 0.21-0.22 s | 0.16-0.17 s with : 0.23-0.26 s | 0.18-0.20 s
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