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Drop logical replication subscribers before compute starts on a non-main branch. Add new compute_ctl spec flag: drop_subscriptions_before_start If it is set, drop all the subscriptions from the compute node before it starts. To avoid race on compute start, use new GUC neon.disable_logical_replication_subscribers to temporarily disable logical replication workers until we drop the subscriptions. Ensure that we drop subscriptions exactly once when endpoint starts on a new branch. It is essential, because otherwise, we may drop not only inherited, but newly created subscriptions. We cannot rely only on spec.drop_subscriptions_before_start flag, because if for some reason compute restarts inside VM, it will start again with the same spec and flag value. To handle this, we save the fact of the operation in the database in the neon.drop_subscriptions_done table. If the table does not exist, we assume that the operation was never performed, so we must do it. If table exists, we check if the operation was performed on the current timeline. fixes: https://github.com/neondatabase/neon/issues/8790