improve async scheduling

[d70-t]

As noted in #12, the async implementation can be slower than the sync implementation in some cases. This should not be.

The MultiGateway sends requests to multiple gateways, trying to balance between them based on how fast they respond and if they explicitly mention that requests are coming in too quick (HTTP status 429). This happens for two purposes: automatic fall back from broken to live gateways and to be nice to (especially public) gateways.

It seems like the current scheduling strategy is not yet very good, leads to bad performance and should be updated.

[d70-t]

With the revised implementation in ipfsspec>=0.5.0, MultiGateway doesn't exist anymore and thus the problem of scheduling is gone as well. Hence, I'll close the issue.