Store deployments in a JSON file

[nlewo]

This MR store previous deployment on the disk, in a JSON file. When, the JSON file is not existing or can't be unmarshaled, it is ignored and replaced.
Currently, we keep the last 10 testing deployments and last 10 main deployments.

Storing the previous deployments on disk will be used to add the following features:

• rollback to previous deployments
• manage gcroots
• block non fast forward checkout (since the last deployed commit id is preserved across comin restart)

[zowoq]

@nlewo Would you be open to having a stateless mode in comin that disables storing deployments and the features that would depend on it?

[nlewo]

@zowoq I would prefer to avoid optional feature because it would make more complex the codebase and the documentation.

However, my goal is to not make comin starting without any state file. If you delete the state file, comin starts correctly (and recreate it).

The size of this file is limited and pretty small (16KB) and it doesn't add any build dependencies (i actually started with a sqlite database but switched to a simple json file).

What is your issue with this file?

[zowoq]

Storing state is really an anti-feature for my use case, basically I just want a more polished version of the autoupgrade module with git polling and prometheus metrics.