[SPARK-12879] [SQL] improve the unsafe row writing framework

As we begin to use unsafe row writing framework(`BufferHolder` and `UnsafeRowWriter`) in more and more places(`UnsafeProjection`, `UnsafeRowParquetRecordReader`, `GenerateColumnAccessor`, etc.), we should add more doc to it and make it easier to use.

This PR abstract the technique used in `UnsafeRowParquetRecordReader`: avoid unnecessary operatition as more as possible. For example, do not always point the row to the buffer at the end, we only need to update the size of row. If all fields are of primitive type, we can even save the row size updating. Then we can apply this technique to more places easily.

a local benchmark shows `UnsafeProjection` is up to 1.7x faster after this PR:
**old version**
```
Intel(R) Core(TM) i7-4960HQ CPU  2.60GHz
unsafe projection:                 Avg Time(ms)    Avg Rate(M/s)  Relative Rate
-------------------------------------------------------------------------------
single long                             2616.04           102.61         1.00 X
single nullable long                    3032.54            88.52         0.86 X
primitive types                         9121.05            29.43         0.29 X
nullable primitive types               12410.60            21.63         0.21 X
```

**new version**
```
Intel(R) Core(TM) i7-4960HQ CPU  2.60GHz
unsafe projection:                 Avg Time(ms)    Avg Rate(M/s)  Relative Rate
-------------------------------------------------------------------------------
single long                             1533.34           175.07         1.00 X
single nullable long                    2306.73           116.37         0.66 X
primitive types                         8403.93            31.94         0.18 X
nullable primitive types               12448.39            21.56         0.12 X
```

For single non-nullable long(the best case), we can have about 1.7x speed up. Even it's nullable, we can still have 1.3x speed up. For other cases, it's not such a boost as the saved operations only take a little proportion of the whole process.  The benchmark code is included in this PR.

Author: Wenchen Fan <wenchen@databricks.com>

Closes #10809 from cloud-fan/unsafe-projection.

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