server: add log for binary execute statement

[jackysp]

What problem does this PR solve?

The dispatch error will print the packet data when meets some errors, e.g.

2018/10/22 15:45:15.528 conn.go:488: [warning] con:1 dispatch error:
id:1, addr:127.0.0.1:56631 status:2, collation:utf8_general_ci, user:root
"\x01\x00\x00\x00\x00\x01\x00\x00\x00\x01\x01\x06\x00"
[table:1048]Column 'i' cannot be null

It is not readable.

What is changed and how it works?

Add additional messages for binary execute statement to make the user easy to know the executing SQL and its parameters.

Check List

Tests

• Manual test (add detailed scripts or steps below)
  Run the following function though binary APIs.

func errMsg(db *sql.DB) {
	var err error
	_, err = db.Exec("drop table if exists t")
	if err != nil {
		fmt.Println(err)
	}
	_, err = db.Exec("create table t (i int not null)")
	if err != nil {
		fmt.Println(err)
	}
	_, err = db.Exec("insert into t values (?)", nil)
	if err != nil {
		fmt.Println(err)
	}
}

it will add the addtional info, e.g.

insert into t values (?) [arguments: <nil>]

Code changes

• Has exported function/method change

Side effects

• Increased code complexity

Need to cherry-pick to release branches.

PTAL @lysu @tiancaiamao

[zz-jason]

LGTM

[zz-jason]

@XuHuaiyu @eurekaka @lysu PTAL

[zz-jason]

@jackysp Do we need to cherry pick it to the release-2.0 and release-2.1 branch?

[jackysp]

Yes. I'll cherry-pick it after it merged. @zz-jason

[jackysp]

/run-all-tests

[XuHuaiyu]

Should we add a comment for this function if we only use it for errorMsg
or
s/ preparedStmt2String/ preparedStmt2ErrMsg

[jackysp]

I think we the function name should describe what it does. This function could also be used in other features, e.g. show processlist, so I think it is not necessary to change the name.

[XuHuaiyu]

LGTM