`conflict.conflicting_change_id` -> `conflict.change_b_id`

[samuelstroschein]

Context

The conflict column names impose a meaning, which is wrong after LIXDK-126.

• conflicting_change_id is the change that conflicts with change_id. After branching, both changes conflict with each other. There is no implication of which change conflicts with another one.

• the column names have been decided before branching where conflicting change id did indeed have a special meaning

Proposal

Change the column names to conflict.change_a_id and conflict.change_b_id to remove meaning.

[samuelstroschein]

blocked until we decide on a conflict solution in branching