Accurate Timestamps on-board

[jclds139]

Wireless usage is somewhat limited because there is no reliable source of timestamps on the boards. There are a few reasons for this:

• For scalability, particularly in harsh environments, we prioritized minimal extra traffic flowing in the Wi-Fi network
• SNTP, the only built-in solution for the ESP8266 we've been using, is only accurate to at most 1 second, so not particularly helpful for the 500 Hz or higher sample rates we typically use
• The current firmware is already pushing the limits of the ESP8266 in terms of RAM usage and operations that can be performed between samples.
• There's no way to read the ESP8266's TSF timer in software