Make scope update speed "sticky"

[tonyc]

Currently, the scope's data speed (aka refresh rate) is always set to "high" when the connection starts. It's been requested/suggested that this value is retained across restarts.

The difficulty is to decide where to set it. The connection speed is set on startup, but it could potentially be changed once the LiveView starts. Otherwise, the bandscope update speed could be stored as part of the connection, but that presents difficulties when changing it in the liveview.