Rename updateFrequency to samplingInterval

[kunlinyu]

According to the description of updateFrequency:

properties:
  updateFrequency:
    description: a time interval of sampling location. The unit is millisecond.
    type: number

If the unit is about time, then the field name should be interval.
If the field name is about frequency, then the unit should be Hz.

We have two options:

1. rename the field name to interval.
2. redefine the unit to Hz.

I decide to choose options 1, rename the field name. Let's consider about existed implementations.
If we redefine the unit, the developer of existed implementation will not get any warning from any IDEs and build systems and linters.
But if we rename the field, they will know that no matter from development tools or runtime errors. So I prefer options 1.