Implement Streaming Mode

[choubacha]

The horizon SDK has a streaming mode:
https://www.stellar.org/developers/horizon/reference/responses.html#streaming

We should test and find out how this plays with the reqwest and hyper work with it. This is a different content type that we'd accept: Accept: text/event-stream and I believe we need to maintain an open connection for it.

I'm not familiar with how this will be handled by reqwest, in hyper it's likely to be handled in a Stream type.

The response coming back from the sync client will be a reader that stays open:

seanmonstar/reqwest#278 (comment)

Implementing a streaming mode may not be available for all endpoints but that doesn't mean we don't need to accept them.