Allow for three way merges to automatically resolve concurrent changes to a JSON object when the changes affect different keys.

[nicktobey]

I recommend looking at one commit at a time.

This PR implemented the "Primary Goal" outlined in this doc.

Basically, when two branches in a merge both modify the same JSON value, but touch different keys, dolt should be able to automatically resolve the merge by doing a Three Way Merge that compares the value on both branches to the value at a common ancestor.

This implementation deviates from the strategy outlined in the doc in one major way: when a branch contains an edit to a nested object, we don't run the diff or merge algorithms recursively. Instead, we generate a diff that contains a JSON path to the modified object. Doing this allows us to generate all the diffs in a single linear pass, and is still easy to reason about what changes are conflicts.

We discussed only performing this merge on JSON objects below a certain size for performance reasons, but the size of the object isn't actually visible to the merge algorithm. Given that this algorithm only runs for rows that would otherwise have a merge conflict, all dolt merges that succeed before this PR should see zero performance impact from this change, so I'm not worried about performance.

[fulghum]

Looks great! Just a few minor comments. Overall, very clean.

[fulghum]

Flag/var to opt-out of json merging looks good!