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
```

bool fIncludeEmpty = false;
    if (!params[1].isNull())
        fIncludeEmpty = params[1].get_bool();
    isminefilter filter = ISMINE_SPENDABLE;
    if(!params[2].isNull())
        if(params[2].get_bool())
            filter = filter | ISMINE_WATCH_ONLY;
    bool has_filtered_address = false;
    CTxDestination filtered_address = CNoDestination();
    if (!by_label && params.size() > 3) {
        if (!IsValidDestinationString(params[3].get_str())) {
            throw JSONRPCError(RPC_WALLET_ERROR, "address_filter parameter was invalid");
        }
        filtered_address = DecodeDestination(params[3].get_str());
        has_filtered_address = true;
    }
    
    std::map<CTxDestination, tallyitem> mapTally;
    for (const std::pair<const uint256, CWalletTx>& pairWtx : pwallet->mapWallet) {
        const CWalletTx& wtx = pairWtx.second;
        if (wtx.IsCoinBase() || !CheckFinalTx(*wtx.tx))
            continue;
        int nDepth = wtx.GetDepthInMainChain(locked_chain);
        if (nDepth < nMinDepth)
            continue;
        for (const CTxOut& txout : wtx.tx->vout)
        {
            CTxDestination address;
            if (!ExtractDestination(txout.scriptPubKey, address))
                continue;