replace uuid4 with 24bit auto-increment base64

Needs:

```sql
DELETE FROM paste;
ALTER TABLE paste MODIFY id MEDIUMINT NOT NULL AUTO_INCREMENT;

DELIMITER @@

DROP PROCEDURE IF EXISTS insert_paste@@
CREATE PROCEDURE insert_paste (
p_content MEDIUMBLOB,
p_raw BIT(1),
OUT p_id MEDIUMINT
)
BEGIN
START TRANSACTION;
INSERT paste (digest, content, raw)
VALUES (UNHEX(SHA1(p_content)), p_content, p_raw);
SELECT last_insert_id() INTO p_id;
COMMIT;
END;
@@

DROP PROCEDURE IF EXISTS get_digest@@
CREATE PROCEDURE get_digest (
p_digest BINARY(20),
OUT p_id MEDIUMINT
)
BEGIN
SELECT id INTO p_id
FROM paste
WHERE digest = p_digest;
END;
@@

DROP PROCEDURE IF EXISTS get_content@@
CREATE PROCEDURE get_content (
p_id MEDIUMINT,
OUT p_content MEDIUMBLOB,
OUT p_raw BIT(1)
)
BEGIN
SELECT content, raw INTO p_content, p_raw
FROM paste
WHERE id = p_id;
END;
@@
```