ddl: placement rules in SQL is not compatible with TiFlash

[xhebox]

Bug Report

When PD schedules a partition to tiflash, TiFlash will validate the keyrange if there is a _r suffix.

But placement rules are not compatible with the old set tiflash replica syntax too, so we should forbid using +engine=tiflash, and should add -engine=tiflash to every rule to avoid schedules to tiflash instance.

1. Minimal reproduce step (Required)

1. tiup playground with at lease 3 tikv instances.

2. add a random placement rule with the label +engine=tiflash. E.g alter table t alter partition p0 add placement policy role=follower replicas=1 constraints='["+engine=tiflash"]'.

3. tiup playground with at lease 3 tikv instances.

4. alter table t set tiflash replica 3

5. alter table t alter partition p0 add placement policy role=follower replicas=1 constraints='["+engine=tiflash"]'.

2. What did you expect to see? (Required)

The first example should work fine. In the 2nd example, 3 should fail.

3. What did you see instead (Required)

TiFlash crashed.

4. What is your TiDB version? (Required)

Newest nightly.

[ti-srebot]

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5. Affected versions

master

6. Fixed versions

master