Migrate dolt checkout to new CLI framework.

[nicktobey]

This PR migrates dolt branch to invoke SQL commands instead of manipulating the database directly. This allows it to work even on remote connections.

We implement this by adding an additional flag to the dolt_checkout stored proceudre: --global. This tells the SQL environment to persist this checkout for subsequent connections, making it the new default branch for future connections.

If a server is currently running, dolt_checkout("--global") will return an error. This is because we try to avoid changing the default branch on a server until we can properly consider the indented effect on existing connections.

Because working sets behave differently in our SQL environment than the command line (command line has a single working set, SQL land has a working set per-branch), we only permit changing the default branch when there would be no observable different in behavior: that is, when both the former and the new branch are clean. If this isn't the case, we return an error. This guarentees that a user calling dolt checkout from the command line will never see behavior that differs from the original CLI implementation. They will, at worst, get a helpful error message.

[macneale4]

Looking good. A couple small style/flow suggestions. Ping me again when the existing and new bats tests are passing.

[macneale4]

Main concern is that we need more tests for testing the state of the current branch when switching between remote and local mode. I'm not convinced that the current local-remote.bash approach covers this well enough.

[macneale4]

Ship when the tests pass!

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