Implement aggregation contract

[bcsainju]

The aggregation contract should be developed and deployed in the icon blockchain.

Its high level functioning will be as:

• Allow configuration to add multiple relayers for different NIDs
• Allow relayers to register new event and make sure duplicate doesn't occur on multiple invocations
• Emit InitialEvent with enough details once any event has been registered
• Perform check on enough relayers acknowledging any packet and emit transfer ready event once enough relayers have acknowledged
• cleanups and required state management

The current assumptions from relayer side are as:

Methods:

// register Packets
registerPacket(byte[] data, BigInteger SN, BigInteger height, String source,String destination,String connAddr)

//acknowlede packet
acknowledgePacket(String source,BigInteger SN,byte[] signedBytes)

//retrieve signature for a packet
byte[][] getSignature(String source,BigInteger SN)

Events:

PacketAcknowledged(BigInteger SN, String source, BigInteger height,byte[] data,String destination)

TransactionReadyToExecute(String destination,BigInteger SN,byte[] data,String source,String connAddr)