Minor follow-up tasks to #1399.

Tasks:

• Replace the simplistic, inefficient VecDeque ringbuffer with the ContGenericRingbuffer, which is much more efficient and better tested. Provision light-client in mutual remote attestation #1399 (comment)
• Write unit tests for the header hash pruning
• Rethink how to handle the locking Provision light-client in mutual remote attestation #1399 (comment)