fix: do not use trace object during processTraceDecisions

[VinozzZ]

Which problem is this PR solving?

Since we have delayed trace decision publication and sending a trace, processTraceDecisions may be processing the same trace as sendTraces which causes concurrent access to the span.Data map.

This PR changes the processTraceDecisions to get trace decision information only from the TraceDecision object and only record a trace decision if it doesn't exist in the decision cache

Short description of the changes

• implement cache.KeptTrace interface for TraceDecision
• only record a trace decision if it doesn't exist in the decision cache