Apply a rehearsed merge to its target

For a merge `failed` in the `online` phase: its transaction is on the target, its online steps didn't all run, and it holds both branches until it is retried to the end or abandoned. Abandoning makes it `cancelled` and frees the branches. Nothing on the target is undone: `result.leftovers` lists what the merge left there that the online phase didn't finish, each `{kind, schema, table, name}`: `not_valid_constraint` (a constraint added `NOT VALID`, never validated; it still checks new rows); `not_null_check` (a temporary `chelabase_merge_nn_*` check that wasn't dropped); `invalid_index` (the index the online phase was building, or dropping concurrently, when it stopped: Postgres may have left it INVALID, unused by queries but still updated by writes; it may also not be there); `unattached_index` (a valid index the merge built that its next step didn't put into place: a temporary `chelabase_merge_ix_*` index not renamed over the one it replaces, or a unique constraint's index not attached to its constraint; nothing uses it, and it costs on every write). Validate, attach or drop them by hand (`DROP INDEX CONCURRENTLY IF EXISTS` for an index).