Summary:
I found that we continue accepting writes even when the blob db goes beyond the configured blob directory size limit. Now, we return an error for writes on reaching `blob_dir_size` limit and if `is_fifo` is set to false. (We cannot just drop any file when `is_fifo` is true.)

Deleting the oldest file when `is_fifo` is true will be handled in a later PR.
Closes https://github.com/facebook/rocksdb/pull/3060

Differential Revision: D6136156

Pulled By: sagar0

fbshipit-source-id: 2f11cb3f2eedfa94524fbfa2613dd64bfad7a23c