add `REPLICAS`, substitute the `FOLLOWERS` keywords in placement rule syntax

## Enhancement
The current syntax is confusing for users, as shown in the following examples:
```
CREATE PLACEMENT POLICY fivereplicas FOLLOWERS=4;
CREATE TABLE t1 (a INT) PLACEMENT POLICY=fivereplicas;
```
To improve clarity and directness, the syntax can be revised to:
```
CREATE PLACEMENT POLICY fivereplicas REPLICAS=5;
CREATE TABLE t1 (a INT) PLACEMENT POLICY=fivereplicas;
```