Add ability to use system time for timestamps

[cboulay]

• Currently SpikeEvent.timestamp is a float but the Blackrock hardware uses int64.
• Legacy NSP's timestamp field is sample-counts since last reset (30,000 Hz)
• Gemini timestamp field is PTP time in nanoseconds
• Most other ezmsg signals / events use system time (time.time()).

We should have a background thread that synchronizes system clock with the hardware clock. This might amount to registering a callback for the heartbeat packets and noting their arrival time, and keeping an SGD regression between the pairs of timestamps.

The clock sync probably only needs to be updated every ~5 seconds.