submission round trip latency

[joelkoen]

Tower Collector can be quite slow when uploading to BeaconDB, I've seen some users submitting for eight minutes straight! There are three main points that significantly contribute to this:

• the BeaconDB submission server is hosted in Australia. Latency from Europe can be >300ms. (the server is moving to Europe, but with VPNs and other high latency networks this might still be a significant issue)
• Tower Collector makes requests synchronously, meaning the client is waiting >300ms per request.
• Tower Collector only sends 80 measurements per request, meaning the client has to make a lot of requests.

As discussed in #232, increasing the amount of measurements to send is difficult to do without a large amount of refactoring, and Tower Collector is designed this way for performance.

Could Tower Collector send multiple requests at once? Any other ideas?

Also, this round trip latency likely won't be as noticeable once I've finished the EU server, so it may not be worth spending too much time on this.