Consistent with other ShareDB repos, and makes sure we run the build
against the latest dependencies possible.

This is a **BREAKING** change that:

 - adds tests against the [upstream `sharedb` DB test suite][1]
 - adds a CI build for the tests against current Node.js and Postgres
   versions
 - breaks the API to conform to the upstream tests, including adding
   metadata support

The breaks are:

 - Dropping non-null constraints on `snapshots.doc_type` and
   `snapshots.data` (to allow `Doc`s to be deleted)
 - Adding a new `snapshots.metadata` `json` column
 - Respecting `options.metadata` and `fields.$submit`, which were
   previously ignored on `getOps()`, and useless on `getSnapshot()`
   (which didn't store metadata)
 - `snapshot.m` is now `undefined` if not present, or `null` if
   unrequested (inline with the spec)

On top of this it also makes some bugfixes to conform to the spec:

 - Ignore unique key validations when committing, since this may happen
   during concurrent commits
 - `JSON.stringify()` JSON fields, which [break][2] if passed a raw
   array
 - Default `from = 0` if unset in `getOps()`

[1]: https://github.com/share/sharedb/blob/7abe65049add9b58e1df638aa34e7ca2c0a1fcfa/test/db.js#L25
[2]: brianc/node-postgres#442

💥 Comply with `sharedb` spec

When updating the snapshot table, the metadata field was mistakenly updated to the data field

Fix snapshot commit metadata on conflict