Chaincode enclave / tlcc binding.

When a new chaincode enclave spawns it binds itself to a given tlcc enclave using local attestation. Typically, this includes a key exchange to protect the communication between the chaincode enclave and tlcc later.

However, the current code base has hardcoded shared key. Attestation and key exchange needs a revision.

Steps
- [x] Design of API and high-level architecture
- [ ] Library implementation
- [ ] quick'n'dirty mock version
- [ ] secure version
- [ ] Library test-cases
- [ ] Library integration
- [ ] Session setup
- from a staging perspective, we might "leak" the key here and stuff it into the place of the currently hard-coded (global) key [note this will prevent concurrent chaincodes with the session lifetime as defined in the UMLs ...)
- [ ] Session request handling

See #410 and #420 for related bigger context.