Refactoring and improved error handling Go/cpp code following #501

[bvavala]

A lot of code has to be refactored both in Go and in the enclave. List of tasks:

• [ ] go pdo crypto in separate module (and cleanup of obsolete internal/utils/crypto.go) to make mock_enclave.go also work with new message format, see comment https://github.com/hyperledger-labs/fabric-private-chaincode/pull/501#discussion_r546065033 and issue #513
• [ ] use of go pdo crypto also for hash, see comment https://github.com/hyperledger-labs/fabric-private-chaincode/pull/501#discussion_r546063031
• [ ] error handling of state functions (they are void, should be bool)
• [ ] state values from string to binary (see comment https://github.com/hyperledger-labs/fabric-private-chaincode/pull/501#discussion_r546074489)