Summary:
Some accesses to blob_files_ and open_ttl_files_ in BlobDBImpl, as well
as to expiration_range_ in BlobFile were not properly synchronized.
The patch fixes this and also makes sure the invariant that obsolete_files_
is a subset of blob_files_ holds even when an attempt to delete an obsolete
blob file fails.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5698

Test Plan:
COMPILE_WITH_TSAN=1 make blob_db_test
gtest-parallel --repeat=1000 ./blob_db_test --gtest_filter="*ShutdownWait*"

The test fails with TSAN errors ~20 times out of 1000 without the patch but
completes successfully 1000 out of 1000 times with the fix.

Differential Revision: D16793235

Pulled By: ltamasi

fbshipit-source-id: 8034b987598d4fdc9f15098d4589cc49cde484e9