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
```

auto locked_chain = pwallet->chain().lock();
    LOCK(pwallet->cs_wallet);
    UniValue obj(UniValue::VOBJ);
    size_t kpExternalSize = pwallet->KeypoolCountExternalKeys();
    obj.pushKV("walletname", pwallet->GetName());
    obj.pushKV("walletversion", pwallet->GetVersion());
    obj.pushKV("balance",       ValueFromAmount(pwallet->GetBalance()));
    obj.pushKV("unconfirmed_balance", ValueFromAmount(pwallet->GetUnconfirmedBalance()));
    obj.pushKV("immature_balance",    ValueFromAmount(pwallet->GetImmatureBalance()));
    obj.pushKV("txcount",       (int)pwallet->mapWallet.size());
    obj.pushKV("keypoololdest", pwallet->GetOldestKeyPoolTime());
    obj.pushKV("keypoolsize", (int64_t)kpExternalSize);
    CKeyID seed_id = pwallet->GetHDChain().seed_id;
    if (pwallet->CanSupportFeature(FEATURE_HD_SPLIT)) {
        obj.pushKV("keypoolsize_hd_internal",   (int64_t)(pwallet->GetKeyPoolSize() - kpExternalSize));
    }
    if (pwallet->IsCrypted()) {
        obj.pushKV("unlocked_until", pwallet->nRelockTime);
    }
    obj.pushKV("paytxfee", ValueFromAmount(pwallet->m_pay_tx_fee.GetFeePerK()));
    if (!seed_id.IsNull()) {
        obj.pushKV("hdseedid", seed_id.GetHex());
    }
    obj.pushKV("private_keys_enabled", !pwallet->IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS));
    return obj;
}
static UniValue listwalletdir(const JSONRPCRequest& request)
{
    if (request.fHelp || request.params.size() != 0) {
        throw std::runtime_error(