@Override
	void invalidateCache() {
		callingConventionNameToIDMap = null;
		callingConventionIDToNameMap = null;
		freeKeySet = null;
	}
Remove next available free key value from freeKeySet.
@return the next available key. A negative value indicates that all allowed IDs have 
been used.
Ghidra/Framework/SoftwareModeling/src/main/java/ghidra/program/database/data/CallingConventionDBAdapterV0.java