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

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)