[YSQL] Support updating primary key columns

[nocaway]

Fix for issue #565 only support UPDATE attributes that are not PRIMARY KEY column.
In YugaByte, updating PRIMARY KEY columns needs special attention because they are currently used for two important purposes.

• Partitioning the table.
• Ordering rows within one table.

[nocaway]

Test case
1- Create a table.
--- CREATE TABLE tab(id int primary key, name text);
2- Insert 100 rows into table.
--- INSERT INTO tab VALUES(x, 'x'); where x = {1, 2, ..., 100 }
3- Update rows
--- UPDATE tab SET id = id + 1000;
4- Turn OFF prefetch by change kPrefetchLimit to 1
--- File pg_doc_op.h: static const int64_t kPrefetchLimit = 1;
--- Also, it's a mistake to use INT32_MAX as default value for number of prefetched rows. We should use "1024" instead.

[sergeyshaykhullin]

+1