Cyclic dependencies in existing modules

## Problem

When I moved the REST service implementation to the `testbench` directory I created a cyclic dependency. Basically `testbench` needs `testbench/rest_server.py` (for the `run()`) function, which needs `gcs/bucket.py`, which needs `testbench` (for all the helper functions).

## Proposal

I think we need to redefine the layers for the testbench, they were already kind of fuzzy, and my breakage makes it worse. I think we should organize the code in the following layers.

| Name | Description | Depends on |
|-----|----|----------|
| `testbench` | The top-level layer, exposes the `run()` function for `gunicorn` and `main` | `testbench.servers` |
| `testbench.servers` | The implementation of the REST and gRPC APIs. Flask routes and gRPC servicers. | `testbench.database`, `testbench.utility`, `google.storage.v2` |
| `testbench.database` | The database of resources (Bucket, Object, *AcccessControl, etc.) | `testbench.utility` |
| `testbench.utility` | Helper functions | `google.storage.v2` |
| `google.storage.v2` | Proto generated files | |

## Where do things move?

The `grpc_server.py` and `rest_server.py` files move to `testbench.servers`. We will *not* split `rest_server.py` as part of this work.

The current `gcs` module would move to `testbench.database`.

For the most part, other `testbench/*.py` files move to `testbench.utility`.

## Breakdown

Some of these are tentative, but the general idea would be to break this down into steps.

- [ ] Move `testbench/error.py` to `testbench/utility/error.py`
- [ ] Split `testbench/common.py` snake case functions to `testbench/utility/*`
- [ ] Move `testbench/acl.py` to `testbench/utility/`
- [ ] Split `testbench/project.py`, move the server-like things to `testbench/servers/` and the rest to `testbench/utility/`.
- [ ] Move `testbench/csek.py` to `testbench/utility/`
- [ ] Move `testbench/generation.py` to `testbench/utility/`
- [ ] Move `testbench/handle_gzip.py` to `testbench/servers/`
- [ ] Move `gcs/*.py` to `testbench/database/*.py`
- [ ] Move `testbench/database.py` to `testbench/database/`
- [ ] Move `testbench/grpc_server.py` to `testbench/servers/`
- [ ] Move `testbench/rest_server.py` to `testbench/servers/`