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
```

{
    std::shared_ptr<CWallet> const wallet = GetWalletForJSONRPCRequest(request);
    CWallet* const pwallet = wallet.get();
    if (!EnsureWalletIsAvailable(pwallet, request.fHelp)) {
        return NullUniValue;
    }
    if (request.fHelp || request.params.size() > 5)
        throw std::runtime_error(
            RPCHelpMan{"listunspent",
                "\nReturns array of unspent transaction outputs\n"
                "with between minconf and maxconf (inclusive) confirmations.\n"
                "Optionally filter to only include txouts paid to specified addresses.\n",
                {
                    {"minconf", RPCArg::Type::NUM,  "1", "The minimum confirmations to filter"},
                    {"maxconf", RPCArg::Type::NUM,  "9999999", "The maximum confirmations to filter"},
                    {"addresses", RPCArg::Type::ARR,  "empty array", "A json array of bitcoin addresses to filter",
                        {
                            {"address", RPCArg::Type::STR, RPCArg::Optional::OMITTED, "bitcoin address"},
                        },
                    },
                    {"include_unsafe", RPCArg::Type::BOOL,  "true", "Include outputs that are not safe to spend\n"
            "                  See description of \"safe\" attribute below."},
                    {"query_options", RPCArg::Type::OBJ, RPCArg::Optional::OMITTED_NAMED_ARG, "JSON with query options",
                        {
                            {"minimumAmount", RPCArg::Type::AMOUNT,  "0", "Minimum value of each UTXO in " + CURRENCY_UNIT + ""},
                            {"maximumAmount", RPCArg::Type::AMOUNT,  "unlimited", "Maximum value of each UTXO in " + CURRENCY_UNIT + ""},
                            {"maximumCount", RPCArg::Type::NUM,  "unlimited", "Maximum number of UTXOs"},