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
```

"    \"blockindex\": n,          (numeric) The index of the transaction in the block that includes it.\n"
            "    \"blocktime\": xxx,         (numeric) The block time in seconds since epoch (1 Jan 1970 GMT).\n"
            "    \"txid\": \"transactionid\",  (string) The transaction id.\n"
            "    \"time\": xxx,              (numeric) The transaction time in seconds since epoch (Jan 1 1970 GMT).\n"
            "    \"timereceived\": xxx,      (numeric) The time received in seconds since epoch (Jan 1 1970 GMT).\n"
            "    \"bip125-replaceable\": \"yes|no|unknown\",  (string) Whether this transaction could be replaced due to BIP125 (replace-by-fee);\n"
            "                                                   may be unknown for unconfirmed transactions not in the mempool\n"
            "    \"abandoned\": xxx,         (bool) 'true' if the transaction has been abandoned (inputs are respendable). Only available for the 'send' category of transactions.\n"
            "    \"comment\": \"...\",       (string) If a comment is associated with the transaction.\n"
            "    \"label\" : \"label\"       (string) A comment for the address/transaction, if any\n"
            "    \"to\": \"...\",            (string) If a comment to is associated with the transaction.\n"
            "  ],\n"
            "  \"removed\": [\n"
            "    <structure is the same as \"transactions\" above, only present if include_removed=true>\n"
            "    Note: transactions that were re-added in the active chain will appear as-is in this array, and may thus have a positive confirmation count.\n"
            "  ],\n"
            "  \"lastblock\": \"lastblockhash\"     (string) The hash of the block (target_confirmations-1) from the best block on the main chain. This is typically used to feed back into listsinceblock the next time you call it. So you would generally use a target_confirmations of say 6, so you will be continually re-notified of transactions until they've reached 6 confirmations plus any new ones\n"
            "}\n"
                },
                RPCExamples{
                    HelpExampleCli("listsinceblock", "")
            + HelpExampleCli("listsinceblock", "\"000000000000000bacf66f7497b7dc45ef753ee9a7d38571037cdb1a57f663ad\" 6")
            + HelpExampleRpc("listsinceblock", "\"000000000000000bacf66f7497b7dc45ef753ee9a7d38571037cdb1a57f663ad\", 6")
                },
            }.ToString());
    
    
    pwallet->BlockUntilSyncedToCurrentChain();
    auto locked_chain = pwallet->chain().lock();
    LOCK(pwallet->cs_wallet);