Prevent enclave from handling events until handshake is complete

[hu55a1n1]

Summary

• The idea is to not run any enclave code until the handshake is complete. e.g. an init_clearing event is not processed until the handshake is complete.
• An additional improvement is to make sure the right handshake msg is processed in a seq (i.e. it's not possible to call session_set_pub_key without calling session_create.

(comment on #174)

Acceptance Criteria

• Events are not processed until the handshake is complete.
• Handshake messages are only processed in the right sequence. (use type-state?)