Summary:
**Context**: We want to roll out the new WAL multiplexed blobstore and need final tweaks before doing that. See https://fb.quip.com/vyTWAgGZSA2Z
Operation key was initially added on D20557702 (3d314761c3) because we had the problem of doing `select * where blobstore_key=X` and getting entries from multiple writings to the same key.
This is not a problem on WAL anymore, because we never do this type of select, each row is enough information for us and there's no need for follow up queries.
After we fully remove old multiplex, we can delete all code for operation key.
Note: Since I was also touching the table schema, I also fixed some columns, made `blob_size` not null, since we never insert a NULL value, it being nullable was a remnant from the old sync queue, where it was added after the table already existed.
Differential Revision: D40718122
fbshipit-source-id: 661c6f28d1c13f94fe757b7c13f5832187146208
a066ea558e