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
```

if (it != pwallet->mapKeyMetadata.end()) {
            meta = &it->second;
        }
    }
    if (!meta) {
        auto it = pwallet->m_script_metadata.find(CScriptID(scriptPubKey));
        if (it != pwallet->m_script_metadata.end()) {
            meta = &it->second;
        }
    }
    if (meta) {
        ret.pushKV("timestamp", meta->nCreateTime);
        if (meta->has_key_origin) {
            ret.pushKV("hdkeypath", WriteHDKeypath(meta->key_origin.path));
            ret.pushKV("hdseedid", meta->hd_seed_id.GetHex());
            ret.pushKV("hdmasterfingerprint", HexStr(meta->key_origin.fingerprint, meta->key_origin.fingerprint + 4));
        }
    }
    
    
    
    UniValue labels(UniValue::VARR);
    std::map<CTxDestination, CAddressBookData>::iterator mi = pwallet->mapAddressBook.find(dest);
    if (mi != pwallet->mapAddressBook.end()) {
        labels.push_back(AddressBookDataToJSON(mi->second, true));
    }
    ret.pushKV("labels", std::move(labels));
    return ret;
}
static UniValue getaddressesbylabel(const JSONRPCRequest& request)