As mentioned in the previous commit, we need to remove the Protobuf
dependency in order to be allowed to import jj into Google's
repo. This commit makes `SimpleOpStore` store its data using Thrift
instead of Protobufs. It also adds automatic upgrade of existing
repos. The upgrade process took 18 s in my repo, which has 22k
operations. The upgraded storage uses practically the same amount of
space. `jj op log` (the full outut) in my repo slowed down from 1.2 s
to 3.4 s. Luckily that's an uncommon operation. I couldn't measure any
difference in `jj status` (loading a single operation).
In order to allow building jj inside of Google, our Protobuf team
doesn't want to us to use a Google-unsupported implementation. Since
there is no supported implementation in Rust, we have to migrate off
of Protobufs. I'm starting with the operation store. This commit moves
the current implementation to a separate file so it can easily be
disabled by a Caargo feature.
