[POSIX] Migrate allows writing checkpoint

[mhutchinson]

This is a pragmatic way to support setting up mirrors (#576) from POSIX logs.
Confirmed this works by cloning SumDB to a POSIX TLogTiles log:

# Run the proxy
go run github.com/transparency-dev/incubator/tree/main/sumdb/cmd@main \
  --listen=":8089" &

# Migrate the data
go run ./cmd/experimental/migrate/posix \
  --storage_dir ~/log-clones/sumdb \
  --source_url http://localhost:8089/ \
  --save_checkpoint

# Verify it mirrored properly
echo sum.golang.org+033de0ae+Ac4zctda0e5eza+HJyk9SxEdh+s3Ux18htTTAD8OuAn8 > ~/.go.sum.vkey
go run github.com/transparency-dev/tessera/cmd/fsck@main \
  --storage_url file:///${HOME}/log-clones/sumdb/ \
  --public_key ~/.go.sum.vkey \
  --origin "go.sum database tree"