Implement Shared Server (a.k.a. Data Collectives, Shared Pod)

See merge request memri/pod!148

Fix integrators callback address

See merge request memri/pod!149

Disallow group imports

See merge request memri/pod!147

For example:
```
use futures::{compat::Compat01As03, future::BoxFuture, prelude::*};
```

and
```
use std::{
    fmt::{self, Display},
    result,
};
```

Implement insert_tree API

See merge request memri/pod!146

Implement date filtering (efficient sync)

See merge request memri/pod!145

Update readme

See merge request !144

Update readme

See merge request memri/pod!143

Add support for server-side dateModified

See merge request memri/pod!142

Previously, if a migration fails the first time,
the second time migrations wouldn't be run.

The second and future HTTP requests would therefore
have not-fully-migrated database.

Forbid underscore in schemas

See merge request memri/pod!141

Use CLI arguments instead of env in docker-compose

See merge request memri/pod!140