Summary:
Best-efforts recovery does not check the content of CURRENT file to determine which MANIFEST to recover from. However, it still checks the presence of CURRENT file to determine whether to create a new DB during `open()`. Therefore, we can tweak the logic in `open()` a little bit so that best-efforts recovery does not rely on CURRENT file at all.

Test plan (dev server):
make check
./db_basic_test --gtest_filter=DBBasicTest.RecoverWithNoCurrentFile
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6970

Reviewed By: anand1976

Differential Revision: D22013990

Pulled By: riversand963

fbshipit-source-id: db552a1868c60ed70e1f7cd252a3a076eb8ea58f