Add `--wait-interval` and `$DBMATE_WAIT_INTERVAL` to make wait interval configurable at runtime

**Description**

When using the `--wait` flag and `dbmate up` is run before the DB is ready, the logs are flooded with this message:

```
[mysql] 2024/01/24 19:58:12 packets.go:37: unexpected EOF
```

Eventually it stops when the DB is ready and it can run migrations:
```
[mysql] 2024/01/24 19:58:12 packets.go:37: unexpected EOF
[mysql] 2024/01/24 19:58:12 packets.go:37: unexpected EOF
Waiting for database.....................................................................................................................
Migrations complete.
Connecting to 127.0.0.1:15020 (127.0.0.1:15020)
```

- Version: 2.11
- Database: mysql
- Operating System: Linux (official Docker container)

**Steps To Reproduce**
Set up a K8S job that will run the dbmate container against a K8S mysql pod. Restart the mysql pod then kick off the dbmate job with `--wait` flag. Observe the log messages.

**Expected Behavior**
I would expect to see the message `Waiting for database` and a `.` for each time it attempts connection until it succeeds.