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
```

Optional<int> height = MakeOptional(false, int()); 
    Optional<int> altheight; 
    int target_confirms = 1;
    isminefilter filter = ISMINE_SPENDABLE;
    uint256 blockId;
    if (!request.params[0].isNull() && !request.params[0].get_str().empty()) {
        blockId = ParseHashV(request.params[0], "blockhash");
        height = locked_chain->findFork(blockId, &altheight);
        if (!height) {
            throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Block not found");
        }
    }
    if (!request.params[1].isNull()) {
        target_confirms = request.params[1].get_int();
        if (target_confirms < 1) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter");
        }
    }
    if (!request.params[2].isNull() && request.params[2].get_bool()) {
        filter = filter | ISMINE_WATCH_ONLY;
    }
    bool include_removed = (request.params[3].isNull() || request.params[3].get_bool());
    const Optional<int> tip_height = locked_chain->getHeight();
    int depth = tip_height && height ? (1 + *tip_height - *height) : -1;
    UniValue transactions(UniValue::VARR);
    for (const std::pair<const uint256, CWalletTx>& pairWtx : pwallet->mapWallet) {
        CWalletTx tx = pairWtx.second;
        if (depth == -1 || tx.GetDepthInMainChain(*locked_chain) < depth) {
            ListTransactions(*locked_chain, pwallet, tx, 0, true, transactions, filter, nullptr );