This is to more accurately match the BLE spec, where intervals are
configured in units of channel hop time (625us). When it was
specified in ms, not all "valid" intervals were able to be
specified.
Now that we're also allowing configuration of scan interval, this
commit updates advertising to match.
This adds two additional optional kwargs to `gap_scan()`:
- `interval_us`: How long between scans.
- `window_us`: How long to scan for during a scan.
The default with NimBLE is a 11.25ms window with a 1.28s interval.
Changing these parameters is important for detecting low-frequency
advertisements (e.g. beacons).
Note: these params are in microseconds, not milliseconds in order
to allow the 625us granularity offered by the spec.
