Add SyncV2 protocol

[ChrisPenner]

Overview

Adds experimental (and hidden) pull.v2 command which uses Share's SyncV2 protocol.

Currently syncv2 is active on both staging and production :)

See caveats in Loose Ends below;
Usage:

$ UNISON_SHARE_HOST=https://staging.api.unison-lang.org stack exec unison -- -C empty
scratch/main> pull.v2 @unison/base/main

  Processed 24858 entities...

  Saved 24858 / 24858 entities...

  ✅

  Successfully pulled into scratch/main, which was empty.

scratch/main>

Implementation notes

• Adds pull.v2 command which streams entities from Share rather than the current iterative deepening approach
• Adds prepared statement caching to SQLite transactions, a test on staging showed an improvement from 20s -> 15s in total sync time for @unison/base/main when statement caching was enabled;
  • TODO: Check whether it's actually the statement caching or just the SQLite tuning that makes a difference here.; Yes it looks like statement caching does make a significat difference (14.5s -> 10s for a unison/base sync)
• Leaves pull unaffected.

Test coverage

I've tested it on staging and everything seems to be working, we'll probably get it running in Nimbus CI or something soon.

There are already some automated tests around the serialization format (from the sync-file PR)

Loose ends

There are several progressive enhancements I'm still working on, none of which should be breaking changes, but best to keep this command experimental for a short while :)

• The current version doesn't handle incremental pulls, e.g. if you pull a branch just to get most-recent changes it'll sync the entire branch's history again (obviously not ideal, I've got a fix in progress)
• Share doesn't yet sort entities in dependency-first order, meaning UCM needs to do an in-memory sort, which slows things down and unfortunately means we need to keep ALL entities in memory during syncing 😬 ; working on a fix for this too :)