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
```

}
    }
    
    
    UniValue removed(UniValue::VARR);
    while (include_removed && altheight && *altheight > *height) {
        CBlock block;
        if (!pwallet->chain().findBlock(blockId, &block) || block.IsNull()) {
            throw JSONRPCError(RPC_INTERNAL_ERROR, "Can't read block from disk");
        }
        for (const CTransactionRef& tx : block.vtx) {
            auto it = pwallet->mapWallet.find(tx->GetHash());
            if (it != pwallet->mapWallet.end()) {
                
                
                ListTransactions(*locked_chain, pwallet, it->second, -100000000, true, removed, filter, nullptr );
            }
        }
        blockId = block.hashPrevBlock;
        --*altheight;
    }
    int last_height = tip_height ? *tip_height + 1 - target_confirms : -1;
    uint256 lastblock = last_height >= 0 ? locked_chain->getBlockHash(last_height) : uint256();
    UniValue ret(UniValue::VOBJ);
    ret.pushKV("transactions", transactions);
    if (include_removed) ret.pushKV("removed", removed);
    ret.pushKV("lastblock", lastblock.GetHex());
    return ret;
}
static UniValue gettransaction(const JSONRPCRequest& request)