Refactoring Message and post model with introduction of meddler

[nak3]

This PR contains refactoring of Message and Post model with:

• Introduction of meddler
• Update of Post model structure
• Move sql statments from sql_stmt.go to post.go

1. Introduction of meddler

1-1. Why meddler?

• Please refer to general motivation in the upstream doc
• Please see how it can decreses codes in message's Save and Select of this PR.

1-2. Feasibility

• This PR contains the update of Post model with medddler. Thus, most probably it is possible to use meddler for other models. Just FYI, but Drone also uses meddler.

2. Update of Post structure

• Post has Tags, User and Comment, so it always pads the data like this. This PR removes them from Post and implements them like this to from handler.

3. Move sql statments from sql_stmt.go into model codes

• Thanks to the meddler, sql statements are not so long anymore. To improve readability, they are moved into their model codes. (e.g) post.go

[nak3]

If this refactoring is accepted, I will continue this for other models.

[dinever]

Hi @nak3, just had a look at this, meddler looks pretty cool. Now we can make our models much simpler. Thank you for your contribution!