improve performance of count_data #1090

Feature Request

Is your feature request related to a problem? Please describe:
When we precisely count data for a large table, it's unnecessarily return key-values from server to client.

Describe the feature you'd like:

I try to add optional section "only_return_count" on get_scanner_request & scan_request, also add optional section kv_count on scan_response.
Different versions of the software will not interact with each other because of optional section.