expression: fix missing microsecond for timestamp

[imtbkcat]

What problem does this PR solve?

Fix #7334 to make JDBC test happy.

What is changed and how it works?

Microsecond missed in add function because date.Fsp become 0 when we use now() and don't set date.Fsp to 6. I add some code to set date.Fsp = 6 if added with microsecond.

Before:

mysql> select now() - interval 1 microsecond;
+--------------------------------+
| now() - interval 1 microsecond |
+--------------------------------+
| 2018-08-16 20:05:19            |
+--------------------------------+

After:

mysql> select now() - interval 1 microsecond;
+--------------------------------+
| now() - interval 1 microsecond |
+--------------------------------+
| 2018-08-16 20:21:01.999999     |
+--------------------------------+

Check List

Tests

• Unit test

Code changes

• Has exported function/method change

Side effects
no

Related changes

[zz-jason]

@imtbkcat Please add some UT

[imtbkcat]

PTAL @zz-jason

[zz-jason]

LGTM

[shenli]

/run-all-tests

[zhexuany]

LGTM

[zimulala]

LGTM