add REPLICAS, substitute the FOLLOWERS keywords in placement rule syntax

[nolouch]

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;

[nikzayn]

I will take this up.

/assign