MDEV-22218 InnoDB: Failing assertion: node->pcur->rel_pos == BTR_PCUR_ON upon LOAD DATA with NO_BACKSLASH_ESCAPES in SQL_MODE and unique blob in table

`inited == NONE` at the initialization time does not always mean
that it'll be `NONE` later, at the execution time. Use a more complex
caller-specific logic to decide whether to create a cloned lookup handler.

Besides LOAD (as in the original bug report) make sure that all
prepare_for_insert() invocations are covered by tests. Add tests for
CREATE ... SELECT, multi-UPDATE, and multi-DELETE.

Don't enable write cache with long uniques.