Summary:
The patch refactors and cleans up the logic around creating new blob files
by moving the common code of `SelectBlobFile` and `SelectBlobFileTTL`
to a new helper method `CreateBlobFileAndWriter`, bringing the implementation
of `SelectBlobFile` and `SelectBlobFileTTL` into sync, and increasing encapsulation
by adding new constructors for `BlobFile` and `BlobLogHeader`.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6066

Test Plan:
Ran `make check` and used the BlobDB mode of `db_bench` to sanity test both
the TTL and the non-TTL code paths.

Differential Revision: D18646921

Pulled By: ltamasi

fbshipit-source-id: e5705a84807932e31dccab4f49b3e64369cea26d