c-s: Implement validation logic for user profiles

[muzarski]

Original c-s allows the user to define custom operations/queries via user command. However, it also predefines two operations. One of them is validate operation. Unfortunately, someone started to work on this, but it does not seem to work during runtime (IIRC, some weird NullPointerException is thrown). We would like to have such logic implemented properly for cql-stress.

Things to do:

• adjust user command CLI, so the users can provide a predefined validate operation in ops parameter. We should also ensure that none of the operations defined in profile yaml is called validate, so there are no naming conflicts.
• implement validate operation. The idea is following:
  • generate a full row for the table (based on the table metadata)
  • fetch a full row from DB (equivalent to SELECT * from table where pk = ?)
  • compare the rows - fail the operation if they are not equal