Improve apply_dml_events_validation batch size handling

[jjh-kim]

Fixed two major issues of apply_dml_events_validation

• When a larger number (higher than apply_dml_events_validation_batch_size) of DML events occur within a given time range, the validator attempts to cut the batch size by reducing the range size. However, when the time range is already 1 second, it can’t reduce the time range further because the binlog event timestamp has a 1-second resolution, which causes it to fall into an infinite loop.
• When creating timestamp ranges (line 282) the while loop condition was batch_start_timestamp < end_timestamp. This had possibility of skipping the end_timestamp when the batch_duration is evaluated as 0

Additionally added feature to not run full_dml_event_validation if full_dml_event_validation is set to 0