The `getaddressesbylabel` API spends a lot more time than the `getaddressesbyaccount` API.

I generated addresses by the `getnewaddress` API, then, I measured time that the `getaddressesby*` API spends. It seems the algorithm of the `getaddressesbylabel` API takes **O(N ^ 2)** time! The results of my survey are as follows.

### getaddressesbylabel

bitcoind v0.17.1 on Ubuntu 16.04 (AWS EC2 t3.large)

#### 11,001 addresses

```sh
time sudo /usr/local/bin/bitcoin-cli -conf=/etc/bitcoin/bitcoin.conf getaddressesbylabel ""
# (omitted)
real    0m1.230s # 1.2s
user    0m0.044s
sys 0m0.048s
```

#### 111,001 addresses

```sh
time sudo /usr/local/bin/bitcoin-cli -conf=/etc/bitcoin/bitcoin.conf getaddressesbylabel ""
# (omitted)
real    1m37.027s # 97.0s
user    0m0.408s
sys 0m0.544s
```

#### 211,001 addresses

```sh
time sudo /usr/local/bin/bitcoin-cli -conf=/etc/bitcoin/bitcoin.conf getaddressesbylabel ""
# (omitted)
real    6m3.727s # 363.7s
user    0m0.800s
sys 0m0.900s
```

### getaddressesbyaccount

bitcoind v0.16.3 on Ubuntu 16.04 (AWS EC2 t3.large)

#### 11,001 addresses

```sh
time sudo /usr/local/bin/bitcoin-cli -conf=/etc/bitcoin/bitcoin.conf getaddressesbyaccount ""
# (omitted)
real    0m0.500s # 0.5s
user    0m0.092s
sys 0m0.160s
```

#### 111,001 addresses

```sh
time sudo /usr/local/bin/bitcoin-cli -conf=/etc/bitcoin/bitcoin.conf getaddressesbyaccount ""
# (omitted)
real    0m1.361s # 1.4s
user    0m0.168s
sys 0m0.348s
```

#### 211,001 addresses

```sh
time sudo /usr/local/bin/bitcoin-cli -conf=/etc/bitcoin/bitcoin.conf getaddressesbyaccount ""
# (omitted)
real    0m2.220s # 2.2s
user    0m0.384s
sys 0m0.532s
```

RPCResult{
            "{\n"
            "  \"name\" :    <wallet_name>,        (string) The wallet name if loaded successfully.\n"
            "  \"warning\" : <warning>,            (string) Warning message if wallet was not loaded cleanly.\n"
            "}\n"
                },
                RPCExamples{
                    HelpExampleCli("loadwallet", "\"test.dat\"")
            + HelpExampleRpc("loadwallet", "\"test.dat\"")
                },
            }.ToString());
    WalletLocation location(request.params[0].get_str());
    if (!location.Exists()) {
        throw JSONRPCError(RPC_WALLET_NOT_FOUND, "Wallet " + location.GetName() + " not found.");
    } else if (fs::is_directory(location.GetPath())) {
        
        fs::path wallet_dat_file = location.GetPath() / "wallet.dat";
        if (fs::symlink_status(wallet_dat_file).type() == fs::file_not_found) {
            throw JSONRPCError(RPC_WALLET_NOT_FOUND, "Directory " + location.GetName() + " does not contain a wallet.dat file.");
        }
    }
    std::string error, warning;
    std::shared_ptr<CWallet> const wallet = LoadWallet(*g_rpc_interfaces->chain, location, error, warning);
    if (!wallet) throw JSONRPCError(RPC_WALLET_ERROR, error);
    UniValue obj(UniValue::VOBJ);
    obj.pushKV("name", wallet->GetName());
    obj.pushKV("warning", warning);
    return obj;
}
static UniValue createwallet(const JSONRPCRequest& request)