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
```

CPubKey master_pub_key;
    if (request.params[1].isNull()) {
        master_pub_key = pwallet->GenerateNewSeed();
    } else {
        CKey key = DecodeSecret(request.params[1].get_str());
        if (!key.IsValid()) {
            throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Invalid private key");
        }
        if (HaveKey(*pwallet, key)) {
            throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Already have this key (either as an HD seed or as a loose private key)");
        }
        master_pub_key = pwallet->DeriveNewSeed(key);
    }
    pwallet->SetHDSeed(master_pub_key);
    if (flush_key_pool) pwallet->NewKeyPool();
    return NullUniValue;
}
void AddKeypathToMap(const CWallet* pwallet, const CKeyID& keyID, std::map<CPubKey, KeyOriginInfo>& hd_keypaths)
{
    CPubKey vchPubKey;
    if (!pwallet->GetPubKey(keyID, vchPubKey)) {
        return;
    }
    KeyOriginInfo info;
    if (!pwallet->GetKeyOrigin(keyID, info)) {
        throw JSONRPCError(RPC_INTERNAL_ERROR, "Internal keypath is broken");
    }
    hd_keypaths.emplace(vchPubKey, std::move(info));
}
UniValue walletprocesspsbt(const JSONRPCRequest& request)