Summary:
The constant `kNoExpiration` is currently defined in an
internal/implementation header (`blob_log_format.h`); the patch moves it
to the public header `blob_db.h` so it is accessible to users.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7018
Test Plan: `make check`
Reviewed By: riversand963
Differential Revision: D22191354
Pulled By: ltamasi
fbshipit-source-id: 98c8012a83b999a3f1a30e955ce6bb71ba29dc5c