rowcodec: fix lose decimal precision during decode new row format to datum

[lysu]

What problem does this PR solve?

tk.MustExec(`drop table if exists t1`)
tk.MustExec(`create table t1(c1 decimal(6,4), primary key(c1))`)
tk.MustExec(`insert into t1 set c1 = 0.1`)
tk.MustExec(`insert into t1 set c1 = 0.1 on duplicate key update c1 = 1`)
tk.MustQuery(`select * from t1 use index(primary)`).Check(testkit.Rows(`1.0000`))

will fail due to lose precision/frac after decode from row data into datum, it will affact for kv get request that need compare datum, like batchGetChecker.

thanks @crazycs520 find this critical bug.

What is changed and how it works?

need set precision/frac for datum just like old decodeOne

and also backport #13837 modification in decode to chunk logic to new row format decode.

Check List

Tests

• Unit test
• Integration test

Code changes

• impl change

Side effects

• n/a

Related changes

• master only

Release note

• n/a.

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This change is 

[lysu]

/run-all-tests

[jackysp]

LGTM

[coocood]

LGTM

[coocood]

/merge

[sre-bot]

/run-all-tests