The keyId field is populated during the signature generation and is used to uniquely identify the key that was used for it.
This change will add a dummy keyId value and a few tests to verify the new format.

1. add int _keyId

member to the SignedLogicalTime class, change its c-tor to take a keyId argument.
2. add a static const SignedLogicalTime::kUninitialized for the default constructed
3. change LogicalTimeMetadata::writeToMetadata to write to logicalTime the keyId as

logicalTime: {
    clusterTime: <highest known valid LogicalTime_MEM>
    signature: {
        hash: HMAC SHA1 hash of cluster time for validation,
        keyId: <int>
    }
}

4. change LogicalTimeMetadata::readFromMetadata to parse the keyId
5. add a test to logical_type_Test.cpp to validate that keyId roundtrip works