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).
`pkgconfig` has been a alias to `pkg-config` since 2021-01-18, and has started being an error since 2022-09-24.
This commit makes recent nixpkgs versions work, but might make version older than January 2021 break (which is probably not an issue).
I was able to build a working musl binary with this change, by running
this command:
```
cargo build --release --target x86_64-unknown-linux-musl
```
Thanks to @arxanas for the tip.
