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 (!request.params[3].isNull()) {
        RPCTypeCheckArgument(request.params[3], UniValue::VBOOL);
        include_unsafe = request.params[3].get_bool();
    }
    CAmount nMinimumAmount = 0;
    CAmount nMaximumAmount = MAX_MONEY;
    CAmount nMinimumSumAmount = MAX_MONEY;
    uint64_t nMaximumCount = 0;
    if (!request.params[4].isNull()) {
        const UniValue& options = request.params[4].get_obj();
        if (options.exists("minimumAmount"))
            nMinimumAmount = AmountFromValue(options["minimumAmount"]);
        if (options.exists("maximumAmount"))
            nMaximumAmount = AmountFromValue(options["maximumAmount"]);
        if (options.exists("minimumSumAmount"))
            nMinimumSumAmount = AmountFromValue(options["minimumSumAmount"]);
        if (options.exists("maximumCount"))
            nMaximumCount = options["maximumCount"].get_int64();
    }
    
    
    pwallet->BlockUntilSyncedToCurrentChain();
    UniValue results(UniValue::VARR);
    std::vector<COutput> vecOutputs;
    {
        auto locked_chain = pwallet->chain().lock();
        LOCK(pwallet->cs_wallet);
        pwallet->AvailableCoins(*locked_chain, vecOutputs, !include_unsafe, nullptr, nMinimumAmount, nMaximumAmount, nMinimumSumAmount, nMaximumCount, nMinDepth, nMaxDepth);
    }
    LOCK(pwallet->cs_wallet);