Network Performance improvements

[remlaps]

Two problems:

• Searching through old posts with a high percentage of inactive authors takes too long under some screening and network conditions.
• API endpoints go offline frequently.

Solution:

1. Implement the ability to switch between API endpoints when one goes offline.
2. Parallelize the search for articles across multiple endpoints (and/or maybe retrieve the candidate articles from SteemWorld/SDS - TBD)

• As a side topic, the general networking logic needs to be rationalized. It was created piece by piece, and should be restructured into a more cohesive design. This restructuring is not a topic for this issue, but if possible, steps in the right direction can/should be taken during implementation.