Summary:
Removed confusing, awkward, and undocumented internal API
ReadOneLine and replaced with very simple LineFileReader.

In refactoring backupable_db.cc, this has the side benefit of
removing the arbitrary cap on the size of backup metadata files.

Also added Status::MustCheck to make it easy to mark a Status as
"must check." Using this, I can ensure that after
LineFileReader::ReadLine returns false the caller checks GetStatus().

Also removed some excessive conditional compilation in status.h

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8026

Test Plan: added unit test, and running tests with ASSERT_STATUS_CHECKED

Reviewed By: mrambacher

Differential Revision: D26831687

Pulled By: pdillinger

fbshipit-source-id: ef749c265a7a26bb13cd44f6f0f97db2955f6f0f