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
```

{
                    {"minconf", RPCArg::Type::NUM,  "1", "The minimum number of confirmations before payments are included."},
                    {"include_empty", RPCArg::Type::BOOL,  "false", "Whether to include labels that haven't received any payments."},
                    {"include_watchonly", RPCArg::Type::BOOL,  "false", "Whether to include watch-only addresses (see 'importaddress')."},
                },
                RPCResult{
            "[\n"
            "  {\n"
            "    \"involvesWatchonly\" : true,   (bool) Only returned if imported addresses were involved in transaction\n"
            "    \"amount\" : x.xxx,             (numeric) The total amount received by addresses with this label\n"
            "    \"confirmations\" : n,          (numeric) The number of confirmations of the most recent transaction included\n"
            "    \"label\" : \"label\"           (string) The label of the receiving address. The default label is \"\".\n"
            "  }\n"
            "  ,...\n"
            "]\n"
                },
                RPCExamples{
                    HelpExampleCli("listreceivedbylabel", "")
            + HelpExampleCli("listreceivedbylabel", "6 true")
            + HelpExampleRpc("listreceivedbylabel", "6, true, true")
                },
            }.ToString());
    
    
    pwallet->BlockUntilSyncedToCurrentChain();
    auto locked_chain = pwallet->chain().lock();
    LOCK(pwallet->cs_wallet);
    return ListReceived(*locked_chain, pwallet, request.params, true);
}
static void MaybePushAddress(UniValue & entry, const CTxDestination &dest)