Deprecate datetime field type in database

[ikedas]

This PR deprecates datetime field type in database. Because:

• There are no portable way to compare, increase or decrease such field values.
• It's slow: Values should be converted to Unix timestamp; Index cannot be applied.

Database will be upgraded automatically during upgrading process.

[racke]

I have some doubts about this idea ... we would have definitely less issues with it when we are using DBIx::Class. The other problem with using Unix timestamps is they are not legible on the SQL level compared to datetime entries.

[ikedas]

I have some doubts about this idea ... we would have definitely less issues with it when we are using DBIx::Class.

I agree to that we would be better to use smarter ORM.

The other problem with using Unix timestamps is they are not legible on the SQL level compared to datetime entries.

Honestly, I wanted to use update_date_* fields to determine either each record have to be removed or not (this is a part of changes related to issue #11), and I found there is no portable way to compare between datetime field and system clock time (we have to support four or more dialects of SQL).

I guess these fields have been used to store just a time stamp and not considered arithmetic usage I wanted.

[ikedas]

For example I want to do such: https://git.io/vAFnj

Anyway, if there are not strong objection, I'd like to merge my PR at this time.