Naïve dependency negotiation API

[ChrisPenner]

Overview

Deployment

• Run SQL migrations/add new functions.

Adds a simple causal-dependencies API for UCM to determine before a pull which hashes it actually needs.

The causal-dependencies endpoint returns a stream of causals to UCM , each tagged with whether it's part of the top-level history of the root, or whether it's a lib root.

UCM can then pick out which of these dependencies it does have, then send those to the server with the pull.v2 request; where Share will cull entire subtrees based on the dependencies it knows UCM has already.

This allows us to use pull.v2 in cases where it's likely to be a small update.

Test coverage

I've tested this locally, I'll throw it up on staging and run it through its paces.

Luckily its' back-compatible with the previous syncv2 version, which simply doesn't do the pre-negotiation.

Loose ends

We may wish to recurse causal-negotiation on lib deps which have missing deps as well. Should be easy enough to include hash-jwts for those;

Or we can do this on the server and include those deps in the initial stream.