Fix error when using SQLite with gRPC-rere

[charlesbvll]

Issue

Description

When starting a flower-server with SQLite, we get an error saying that SQLite objects created in a specific thread can only be used in that thread.

Related issues/PRs

N/A

Proposal

Explanation

The issue is caused by the fact that we pass the same SQLite State object to all the FleetServicer functions that are called on different threads. Instead, we should create the State object in each of the FleetServicer functions.

Checklist

• Implement proposed change
• Update the changelog entry below
• Make CI checks pass
• Ping maintainers on Slack (channel #contributions)

Changelog entry

Any other comments?

Should this be an extensive changelog entry?