tuple: remove some dependencies on struct tuple_update and rope

Function do_op_insert() was depending on struct tuple_update
because of 'region' field. In next patches struct tuple_update
will be on much more high level of API than do_op_insert and such
a dependency would be wrong. Drop it.

Functions update_read_ops(), update_finish(),
tuple_upsert_squash() were depending on update_alloc() function
intended to be used by rope only. Rope will migrate down to lower
API in next patches, so this dependency should not exist.

Part of #1261