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
```

obj.pushKV("sigsrequired", solutions_data[0][0]);
            UniValue pubkeys(UniValue::VARR);
            for (size_t i = 1; i < solutions_data.size() - 1; ++i) {
                CPubKey key(solutions_data[i].begin(), solutions_data[i].end());
                if (include_addresses) a.push_back(EncodeDestination(key.GetID()));
                pubkeys.push_back(HexStr(key.begin(), key.end()));
            }
            obj.pushKV("pubkeys", std::move(pubkeys));
        }
        
        
        
        
        if (include_addresses) obj.pushKV("addresses", std::move(a));
    }
    explicit DescribeWalletAddressVisitor(CWallet* _pwallet) : pwallet(_pwallet) {}
    UniValue operator()(const CNoDestination& dest) const { return UniValue(UniValue::VOBJ); }
    UniValue operator()(const CKeyID& keyID) const
    {
        UniValue obj(UniValue::VOBJ);
        CPubKey vchPubKey;
        if (pwallet && pwallet->GetPubKey(keyID, vchPubKey)) {
            obj.pushKV("pubkey", HexStr(vchPubKey));
            obj.pushKV("iscompressed", vchPubKey.IsCompressed());
        }
        return obj;
    }
    UniValue operator()(const CScriptID& scriptID) const