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
```

},
            }.ToString());
    
    
    pwallet->BlockUntilSyncedToCurrentChain();
    auto locked_chain = pwallet->chain().lock();
    LOCK(pwallet->cs_wallet);
    RPCTypeCheckArgument(request.params[0], UniValue::VBOOL);
    bool fUnlock = request.params[0].get_bool();
    if (request.params[1].isNull()) {
        if (fUnlock)
            pwallet->UnlockAllCoins();
        return true;
    }
    RPCTypeCheckArgument(request.params[1], UniValue::VARR);
    const UniValue& output_params = request.params[1];
    
    std::vector<COutPoint> outputs;
    outputs.reserve(output_params.size());
    for (unsigned int idx = 0; idx < output_params.size(); idx++) {
        const UniValue& o = output_params[idx].get_obj();
        RPCTypeCheckObj(o,
            {
                {"txid", UniValueType(UniValue::VSTR)},
                {"vout", UniValueType(UniValue::VNUM)},
            });
        const uint256 txid(ParseHashO(o, "txid"));
        const int nOutput = find_value(o, "vout").get_int();
        if (nOutput < 0) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, vout must be positive");