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
```

},
                RPCExamples{
            "\nGenerate 11 blocks\n"
            + HelpExampleCli("generate", "11")
                },
            }.ToString());
    }
    if (!IsDeprecatedRPCEnabled("generate")) {
        throw JSONRPCError(RPC_METHOD_DEPRECATED, "The wallet generate rpc method is deprecated and will be fully removed in v0.19. "
            "To use generate in v0.18, restart bitcoind with -deprecatedrpc=generate.\n"
            "Clients should transition to using the node rpc method generatetoaddress\n");
    }
    int num_generate = request.params[0].get_int();
    uint64_t max_tries = 1000000;
    if (!request.params[1].isNull()) {
        max_tries = request.params[1].get_int();
    }
    std::shared_ptr<CReserveScript> coinbase_script;
    pwallet->GetScriptForMining(coinbase_script);
    
    if (!coinbase_script) {
        throw JSONRPCError(RPC_WALLET_KEYPOOL_RAN_OUT, "Error: Keypool ran out, please call keypoolrefill first");
    }
    
    if (coinbase_script->reserveScript.empty()) {
        throw JSONRPCError(RPC_INTERNAL_ERROR, "No coinbase script available");
    }
    return generateBlocks(coinbase_script, num_generate, max_tries, true);
}
UniValue rescanblockchain(const JSONRPCRequest& request)