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
```

HelpRequiringPassphrase(pwallet) + "\n",
                {
                    {"newsize", RPCArg::Type::NUM,  "100", "The new keypool size"},
                },
                RPCResults{},
                RPCExamples{
                    HelpExampleCli("keypoolrefill", "")
            + HelpExampleRpc("keypoolrefill", "")
                },
            }.ToString());
    if (pwallet->IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS)) {
        throw JSONRPCError(RPC_WALLET_ERROR, "Error: Private keys are disabled for this wallet");
    }
    auto locked_chain = pwallet->chain().lock();
    LOCK(pwallet->cs_wallet);
    
    unsigned int kpSize = 0;
    if (!request.params[0].isNull()) {
        if (request.params[0].get_int() < 0)
            throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, expected valid size.");
        kpSize = (unsigned int)request.params[0].get_int();
    }
    EnsureWalletIsUnlocked(pwallet);
    pwallet->TopUpKeyPool(kpSize);
    if (pwallet->GetKeyPoolSize() < kpSize) {
        throw JSONRPCError(RPC_WALLET_ERROR, "Error refreshing keypool.");
    }
    return NullUniValue;
}
static UniValue walletpassphrase(const JSONRPCRequest& request)