Get balances API endpoint

[GET] /api/balances?take=integer&[continuation=string]

Should return balances of the observed wallets with non zero balances. Wallets balance observation is enabled by the `[POST] /api/balances/{address}/observation` and disabled by the `[DELETE] /api/balances/{address}/observation`.

Amount of the returned balances should not exceed `take` . Optional continuation contains context of the previous request, to let Blockchain.Api resume reading of the balances from the previous position. If continuation is empty, balances should be read from the beginning.

Response:

```js
{

{
// Flag, which indicates that the address is valid
“isValid”: boolean,
// Continuation token, that
// can be used to continue data reading
// from the current position.
// Should be null or empty string if no more data
// to read.
“continuation”: “string”,
// Current batch of the items
// Should be empty array if there are no items
“items”:
[
{
// Wallet address
“address”: “string”,
// Asset ID , defaults to SKY
“assetId”: “string”,
// Balance is integer as string, aligned
// to the asset accuracy. Actual value can be
// calculated as
// x = sourceBalance * (10 ^ asset.Accuracy)
“balance”: “string”,
// Incremental ID of the moment, when balance
// is updated. It should be the same sequence
// as for block in the
// [GET] /api/transactions/broadcast/* responses
// In other words , block number/height.
“block”: integer64
}
] }
```

Since there are no constraints on tokens returned for continuation , some performance improvements should be obtained by returning the last address itself as continuation token for pagination. Should be stored in Redis index .

Python implementation

```py

@api.route('/balances', methods=['GET'])
def get_balances():
"""
Get balances of addresses in observation list
"""

update_index()

take = request.args.get('take')
if take is None:
take = app_config.DEFAULT_LIST_LENGTH
else:
take = max(int(take), app_config.DEFAULT_LIST_LENGTH)

continuation = request.args.get('continuation') #Holds the continuation address previously sent to client
if continuation is None:
continuation = ""

#Get address list from mongodb
addresses = get_addresses_balance_observation()

items = []

#define search boundaries
start_index = 0 if continuation == "" or continuation not in addresses else addresses.index(continuation)
total_items = take if take != 0 else len(addresses)

blockheight = get_indexed_blockheight()
if 'error' in blockheight:
return make_response(jsonify(build_error(blockheight["error"])), blockheight["status"])

while len(items) < total_items and start_index < len(addresses):
item = {}

#Get balance from index
balance = get_indexed_balance(addresses[start_index])
if 'error' in balance: #If there is an error in balance, continue with the next address
start_index += 1
continue

#Generate output response
item['address'] = addresses[start_index]
item['assetId'] = app_config.SKYCOIN_FIBER_ASSET
item['balance'] = str(balance['balance']) #TODO: Asset accuracy. Find endpoint for that
item['block'] = blockheight['blockheight']
if balance['balance'] != 0:
items.append(item)

start_index += 1

#Add continuation address
if start_index < len(addresses): #Still data to read
#If it is the first call and need continuation create the token
if continuation == "" and take != 0 and take < len(addresses):
continuation = addresses[start_index]
else:
continuation = ""

response = {"continuation": continuation, "items": items}

return jsonify(response)
```