cubicweb #1625257 speed up multiple relation setup [resolved]

in the same fashion as the optimisation for multiple entity deletion, add support for optimized multiple relation creation.

prioritynormal
typetask
done in3.12.3
load left0.000
closed by#576abb8c4626 fix implementation of repository.glob_add_relations (closes ##1625257), #9f619715665b [server] improve the speed of setting relations between entities (closes #1625257)