Simplify BcoinClient's `getMerkleProof`

[ErinHales]

In getMerkleProof, it builds the merkle tree twice (once inside merkle.createRoot and once inside merkle.createBranch. I think this could be simplified to where this only happens once.

async getMerkleProof(txid, height) {
    const block = await super.execute('getblockbyheight', [height]);

    let index = -1;
    const txs = [];
    for (const [i, tx] of Object.entries(block.tx)) {
      if (tx === txid) { index = i >>> 0; } // cast to uint from string
      txs.push(Buffer.from(tx, 'hex').reverse());
    }

    assert(index >= 0, 'Transaction not in block.');

    const [root] = merkle.createRoot(hash256, txs.slice());
    assert.bufferEqual(Buffer.from(block.merkleroot, 'hex').reverse(), root);

    const branch = merkle.createBranch(hash256, index, txs.slice());

    const proof = [];
    for (const hash of branch) { proof.push(hash.toString('hex')); }

    return [proof, index];
  }