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{
            "\nThe total amount in the wallet with 1 or more confirmations\n"
            + HelpExampleCli("getbalance", "") +
            "\nThe total amount in the wallet at least 6 blocks confirmed\n"
            + HelpExampleCli("getbalance", "\"*\" 6") +
            "\nAs a JSON-RPC call\n"
            + HelpExampleRpc("getbalance", "\"*\", 6")
                },
            }.ToString());
    
    
    pwallet->BlockUntilSyncedToCurrentChain();
    auto locked_chain = pwallet->chain().lock();
    LOCK(pwallet->cs_wallet);
    const UniValue& dummy_value = request.params[0];
    if (!dummy_value.isNull() && dummy_value.get_str() != "*") {
        throw JSONRPCError(RPC_METHOD_DEPRECATED, "dummy first argument must be excluded or set to \"*\".");
    }
    int min_depth = 0;
    if (!request.params[1].isNull()) {
        min_depth = request.params[1].get_int();
    }
    isminefilter filter = ISMINE_SPENDABLE;
    if (!request.params[2].isNull() && request.params[2].get_bool()) {
        filter = filter | ISMINE_WATCH_ONLY;
    }
    return ValueFromAmount(pwallet->GetBalance(filter, min_depth));
}
static UniValue getunconfirmedbalance(const JSONRPCRequest &request)