Optimize the ttl grammar for label the ttl_col , alter grammer.

[Shylock-Hg]

What changes were proposed in this pull request?

Change the grammar about ttl, not change the inner implement just modify the parser.

1. Change ttl_col from string to label typed.
   such as
   from

create tag tag1(id int, name string) ttl_col = "id", ttl_duration = 1

to

create tag tag1(id int, name string) ttl_col = id, ttl_duration = 1

2. Alter grammar
   drop from

alter tag tag1 ttl_col = ""

to

alter tag tag1 drop ttl

change from

alter tag tag1 ttl_col = 'id', ttl_duration = 444

to

alter tag tag1 change ttl_col = id, ttl_duration = 444

Why are the changes needed?

1. Avoid using string in label place to reduce user confuse
2. Avoid assign statement represent drop semantic

Does this PR introduce any user-facing change?

Yes. @Amber1990Zhang

How was this patch tested?

[panda-sheep]

Good job, but I don't feel very friendly to user
alter tag t drop ttl; will make the user feel that this is deleting a column, the column name is ttl

[Shylock-Hg]

Good job, but I don't feel very friendly to user
alter tag t drop ttl; will make the user feel that this is deleting a column, the column name is ttl

Delete the column is different.

[laura-ding]

The ttl_col is the table properpty, I prefer to stay the same or add a new keyword.
such as SET.

[laura-ding]

I don't think it's necessary to add the keyword KW_TTL

[Shylock-Hg]

Yes, but drop ttl_col sounds like drop the schema field.

[Shylock-Hg]

The ttl_col is the table properpty, I prefer to stay the same or add a new keyword.
such as SET.

Such as alter tag tag1 set ttl_col = label?

[laura-ding]

The ttl_col is the table properpty, I prefer to stay the same or add a new keyword.
such as SET.

Such as alter tag tag1 set ttl_col = label?

Yes!

[dangleptr]

I don't think current changes is good. We could take mysql "alter table" as our reference.

[Shylock-Hg]

A better way in #1922