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
```

}
    LOCK(pwallet->cs_wallet);
    UniValue ret(UniValue::VOBJ);
    CTxDestination dest = DecodeDestination(request.params[0].get_str());
    
    if (!IsValidDestination(dest)) {
        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Invalid address");
    }
    std::string currentAddress = EncodeDestination(dest);
    ret.pushKV("address", currentAddress);
    CScript scriptPubKey = GetScriptForDestination(dest);
    ret.pushKV("scriptPubKey", HexStr(scriptPubKey.begin(), scriptPubKey.end()));
    isminetype mine = IsMine(*pwallet, dest);
    ret.pushKV("ismine", bool(mine & ISMINE_SPENDABLE));
    bool solvable = IsSolvable(*pwallet, scriptPubKey);
    ret.pushKV("solvable", solvable);
    if (solvable) {
       ret.pushKV("desc", InferDescriptor(scriptPubKey, *pwallet)->ToString());
    }
    ret.pushKV("iswatchonly", bool(mine & ISMINE_WATCH_ONLY));
    UniValue detail = DescribeWalletAddress(pwallet, dest);
    ret.pushKVs(detail);
    if (pwallet->mapAddressBook.count(dest)) {
        ret.pushKV("label", pwallet->mapAddressBook[dest].name);
    }
    ret.pushKV("ischange", pwallet->IsChange(scriptPubKey));
    const CKeyMetadata* meta = nullptr;
    CKeyID key_id = GetKeyForDestination(*pwallet, dest);
    if (!key_id.IsNull()) {
        auto it = pwallet->mapKeyMetadata.find(key_id);