Lazy runtime CORs checks

[zone117x]

Once https://github.com/blockstack/gaia/pull/276 is merged, the two additional CORs configs (one for the read-endpoint, one for the write-endpoint) are likely sources of confusion for anyone trying to setup a functioning Gaia hub.

This could be alleviated by a background function -- ran once on hub startup -- that performs a check to determine likely incorrect CORs configurations, and console.error with a helpful message.

Due to variance in how cloud storage drivers and Gaia can be configured, this check cannot be 100% accurate for all possible Gaia setups. This means the console error message must be something like:

Detected likely CORs configuration problems: <details...>. 
This message can be ignored if you know what you are doing.

Alternatively, this detection could process.exit() with an error message along the lines of:

Detected likely CORs configuration problems <details...>. 
If you know what you are doing, this check can be disabled by specifying 
`ignoreCorsCheck=true` in config.json or env vars. 

Note: an ignoreCorsCheck option should be implemented regardless of the above error handling. Serverless providers which perform a hub startup on every http request will want to disable this option.

A complimentary CORs check could also be performed by having the server initiate a fetch request against both the read and write endpoints, using the same CORs web browser restrictions as a client fetch request. In some ways, this check is a better indicator than driver-specific checks. It also can be a generalized function without any driver-specific code. However, this still only determines potential likely problems -- gaia hubs can be configured ways that make it impossible to internally perform the same http requests that an actual client ends up performing.

👍 @kantai for the idea