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 memri/pod!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

Implement CLI support

See merge request memri/pod!138

Clarify sha256 file upload

See merge request memri/pod!139

Fix docs

See merge request memri/pod!137

Make release, update dependencies

See merge request memri/pod!135

Importer integration

See merge request memri/pod!134

Improve services logging

See merge request memri/pod!133

Improve/fix environment variables for Pod/services

See merge request !132

This will differentiate the address on which Pod listens to requests,
and the Pod address that services will see in order to make the callback.

This commit only formalizes the difference and makes it clearer,
it was partly already the case before.

Additionally, default port value of 3030 was moved to a constant,
and docs were improved.