Should not allow specify FOLLOWERS/LEARNERS when FOLLOWER_CONSTRAINTS/LEARNER_CONSTRAINTS is using dict syntax

[lcwangchao]

We should forbid the following syntax:

create placement policy p1 LEARNER_CONSTRAINTS="{+region=r1: 1}" LEARNERS=1;
create placement policy p1 FOLLOWER_CONSTRAINTS="{+region=r1: 1}" FOLLOWERS=1;
create table t1(id int) LEARNER_CONSTRAINTS="{+region=r1: 1}" LEARNERS=1;
create table t1(id int) FOLLOWER_CONSTRAINTS="{+region=r1: 1}" FOLLOWERS=1;