Summary:
There is no systematic mechanism to prevent BlockBasedTableBuilder's status to be set from non-OK to OK. Adding a mechanism to force this will help us prevent failures in the future.

The solution is to only make it possible to set the status code if the status code to set is not OK.

Since the status code passed to CompressAndVerifyBlock() is changed, a mini refactoring is done too so that the output arguments are changed from reference to pointers, based on Google C++ Style.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6776

Test Plan: Run all existing test.

Reviewed By: pdillinger

Differential Revision: D21314382

fbshipit-source-id: 27000c10f1e4c121661e026548d6882066409375