Smart portal selection

[mrcnski]

Smart Portal Selection

Eventually we want to be smarter with portal selection across our SDKs. This means we will need to extend the client model to all of our SDKs so that we can perform the portal selection only once, when initializing the client. Then we want to implement some checks to select the best portal in the absence of user/dev input.

Design

The priority should be:

1. Prefer user-configured portal (the portal hosting the sky app) (skynet-js only)
2. Prefer dev-configured portal (option passed to the Client constructor)
3. Prefer siasky.net

We also may want to factor in locally-running siad instances as well as user configuration (dotfile or a cookie).

We will need to test for missing capabilities (min version, feature support) for each portal picked. We should hit an endpoint like /version (we might want to make this more specific like skynet-capabilities).

This will also catch the case in point 1 where we are running a web app that is not a sky app on a portal.

When initiating a client we may want to assume that it'll fail, so we'll actually ping the:

• current portal
• dev portal
• siasky portal

all at once, and then use the best one that doesn't return an error or a response that indicates missing capabilities.