Add async batch

[IvanIvanoff]

Multiple queries can be executed in a batch to speed up the performance.

There are two batch classes provided - Batch and AsyncBatch.

• Batch combines all the provided queries in a single GraphQL document and executes them in a single HTTP request. This batching technique should be used when lightweight queries that don't fetch a lot of data are used. The reason is that the complexity of each query is accumulated and the batch can be rejected.
• AsyncBatch executes all the queries in separate API calls. The benefit of using AsyncBatch over looping and executing every API call is that the queries can be executed concurrently. The concurrency is controlled by the max_workers optional parameter to the execute function. By default the max_workers value is 10.

Note: If you have been using Batch() and want to switch to the newer AsyncBatch() you only need to change the batch initialization. The functions for adding queries and executing the batch, as well as the format of the response, are the same.

from san import AsyncBatch

batch = AsyncBatch()

batch.get(
    "daily_active_addresses/santiment",
    from_date="2018-06-01",
    to_date="2018-06-05",
    interval="1d"
)
batch.get(
    "transaction_volume/santiment",
    from_date="2018-06-01",
    to_date="2018-06-05",
    interval="1d"
)
[daa, trx_volume] = batch.execute(max_workers=10)

[robcaulk]

This is a very needed feature - thank you for implementing.