Add burn transaction

[jasny]

The JUICY upgrade has added a new transaction type: Burn (type 21). A burn transaction is similar to a transfer transaction, but there is no recipient. The amount is deducted from the sender's balance, and removed from the total supply (aka burned). Burn transactions should be supported by the java client.

The binary schema and JSON are described in the documentation. This transaction type is already implemented in Python, which can be used as example.

• Add Burn class to the transaction package (com.ltonetwork.client.core.transaction).
• Copy logic from other classes where it applies. The class properties should match those available in the burn JSON.
• Implement the toBinaryV3() method using information from the documentation and/or python lib. (Note that there is no v1 or v2 binary for burn transactions.)
• Create unit tests for this class.