FCPInterface.handleGetTrustersCount(): Document why we synchronize

This commit would have added synchronization similar to the preceding
commits, but the code already contains the necessary synchronization.
Thus, only add a comment which explains why it is there.
FCPInterface.handleGetTrusters(): Document why we synchronize

This commit would have added synchronization similar to the preceding
commits, but the code already contains the necessary synchronization.
Thus, only add a comment which explains why it is there.
FCPInterface.handleGetTrustees(): Document why we synchronize

This commit would have added synchronization similar to the preceding
commits, but the code already contains the necessary synchronization.
Thus, only add a comment which explains why it is there.