JsonRpcServer: Add method to get an accounts tree chunk

Add RPC method to get an accounts tree chunk: `getAccountsTreeChunk`.
The method receives as `string` arguments` the block hash on which
the snapshot will be taken and the `startPrefix` of the accounts tree
chunk. Then it returns an `AccountsTreeChunk` object.

clients/nodejs/modules/JsonRpcServer.js

@@ -127,6 +127,7 @@ class JsonRpcServer {
         this._methods.set('createAccount', this.createAccount.bind(this));
         this._methods.set('getBalance', this.getBalance.bind(this));
         this._methods.set('getAccount', this.getAccount.bind(this));
+        this._methods.set('getAccountsTreeChunk', this.getAccountsTreeChunk.bind(this));
 
         // Blockchain
         this._methods.set('blockNumber', this.blockNumber.bind(this));
@@ -593,6 +594,11 @@ class JsonRpcServer {
         return this._accountToObj(account, address);
     }
 
+    async getAccountsTreeChunk(blockHash, startPrefix) {
+        const chunk = await this._consensus.blockchain.getAccountsTreeChunk(Nimiq.Hash.fromString(blockHash), startPrefix);
+        return chunk ? this._accountsTreeChunkToObj(chunk) : null;
+    }
+
     /*
      * Blockchain
      */
@@ -718,6 +724,31 @@ class JsonRpcServer {
         return obj;
     }
 
+    /**
+     * @param {AccountsTreeNode} node
+     * @returns {object}
+     * @private
+     */
+    _accountsTreeNodeObj(node) {
+        return {
+            prefix: node.prefix,
+            account: this._accountToObj(node.account, Nimiq.Address.fromString(node.prefix))
+        }
+    }
+
+    /**
+     * @param {AccountsTreeChunk} chunk
+     * @returns {object}
+     * @private
+     */
+    _accountsTreeChunkToObj(chunk) {
+        return {
+            nodes: chunk.terminalNodes.filter((node) => node.isTerminal()).map((node) => this._accountsTreeNodeObj(node)),
+            proof: chunk.proof.toString(),
+            tail: chunk.tail.prefix,
+        };
+    }
+
     /**
      * @param {Client.PeerInfo} peerInfo
      * @param {Client.AddressInfo} addressInfo