Make the rate more predictable

[hariso]

Feature description

The current approach with read time won't exactly produce the number of messages per second. It will always sleep for the specified time before producing the record, regardless of how much time elapsed since the last time the connector produced a record. Using something like golang.org/x/time/rate would make this more accurate. Another improvement we could do if/when we have time.

-- @lovromazgon