*: remove fsp out of Datum and Duration(experimental)

[lysu]

What have you changed? (mandatory)

The purpose of this PR is remove fsp from Time(types.Duration).

What's Fsp?

For MySQL, Time/Timestamp/Datetime has an fsp argument is given to specify a fractional seconds precision from 0 to 6, the return value includes a fractional seconds part of that many digits.
https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html

what's the problem now we have?

In current version(2.1 beta), types.Duration(stand for MySQL Time) has combinate time.Duration with fsp(int) in types.Duration

but after reading previous MySQL document, we can abstract fsp is just :

• an Column definition
• an Expression part(in builtin)
• and it's real effection is control the digit nums of return value for Result column

So...fsp seems not property of Data type(like Time), but property of

• Column definition
• Expression property
• Derived Result column type(this is done by Huaiyu last year)

but NOT a property of data , it will be only be calculate & allocate one time for a query in type infer stage, no need calcuate it in every row.

How?

to solve this, we should

• remove fsp, and make types.Duration be just a alias of time.Duration(long), make arrow use 8bytes instead of 16bytes
• modify types.Duration output method and rely a input parameter to determine (fsp) during output stage
  pass ResultType.Column.decimal as new added parameter value
• modify all time related function and testcase to make type decimals infer work well.

What are the type of the changes (mandatory)?

At, first we remove fsp in types.Duration, and modify ToString to let fsp can be pass in different context.

Then modify time related funcation and testcase to meet type decimal infer.

How has this PR been tested (mandatory)?

• unit tests
• integration tests

---

This change is 

[jackysp]

Please add comments for this.

[lysu]

/run-all-tests