FFM-12192 Use client Singleton / Lock eval & target map clone operation

[erdirowlands]

What

• We are seeing segment faults that are closely linked to the evaluation metrics map.
• Customer shared logs to confirm the evaluation metrics map has stored corrupt data.

Theory

Theory is when we clone the evaluation metrics map, the evaluation thread can and will modify the original map concurrently, resulting in data corruption. Theory extends to the Segment fault being caused by this access too.

Also, the cf_client uses a class variable which is not thread safe, which could be causing issues.

Fix

Lock the cloning operation with a mutex
Replace the client class variable with a Singleton

Testing

• Unit tests for singleton access
• Testgrid