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
```

const CTxDestination& address = item_it->first;
        const std::string& label = item_it->second.name;
        auto it = mapTally.find(address);
        if (it == mapTally.end() && !fIncludeEmpty)
            continue;
        CAmount nAmount = 0;
        int nConf = std::numeric_limits<int>::max();
        bool fIsWatchonly = false;
        if (it != mapTally.end())
        {
            nAmount = (*it).second.nAmount;
            nConf = (*it).second.nConf;
            fIsWatchonly = (*it).second.fIsWatchonly;
        }
        if (by_label)
        {
            tallyitem& _item = label_tally[label];
            _item.nAmount += nAmount;
            _item.nConf = std::min(_item.nConf, nConf);
            _item.fIsWatchonly = fIsWatchonly;
        }
        else
        {
            UniValue obj(UniValue::VOBJ);
            if(fIsWatchonly)
                obj.pushKV("involvesWatchonly", true);
            obj.pushKV("address",       EncodeDestination(address));
            obj.pushKV("amount",        ValueFromAmount(nAmount));
            obj.pushKV("confirmations", (nConf == std::numeric_limits<int>::max() ? 0 : nConf));
            obj.pushKV("label", label);