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
```

+ HelpExampleCli("listunspent", "6 9999999 '[]' true '{ \"minimumAmount\": 0.005 }'")
            + HelpExampleRpc("listunspent", "6, 9999999, [] , true, { \"minimumAmount\": 0.005 } ")
                },
            }.ToString());
    int nMinDepth = 1;
    if (!request.params[0].isNull()) {
        RPCTypeCheckArgument(request.params[0], UniValue::VNUM);
        nMinDepth = request.params[0].get_int();
    }
    int nMaxDepth = 9999999;
    if (!request.params[1].isNull()) {
        RPCTypeCheckArgument(request.params[1], UniValue::VNUM);
        nMaxDepth = request.params[1].get_int();
    }
    std::set<CTxDestination> destinations;
    if (!request.params[2].isNull()) {
        RPCTypeCheckArgument(request.params[2], UniValue::VARR);
        UniValue inputs = request.params[2].get_array();
        for (unsigned int idx = 0; idx < inputs.size(); idx++) {
            const UniValue& input = inputs[idx];
            CTxDestination dest = DecodeDestination(input.get_str());
            if (!IsValidDestination(dest)) {
                throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, std::string("Invalid Bitcoin address: ") + input.get_str());
            }
            if (!destinations.insert(dest).second) {
                throw JSONRPCError(RPC_INVALID_PARAMETER, std::string("Invalid parameter, duplicated address: ") + input.get_str());
            }
        }
    }
    bool include_unsafe = true;