proxy: do not do flush if the coming CompactLog is doomed to fail

Enhancement

exec_compact_log can fail for the following reasons which may be common in some TiFlash's cases.

...
        if compact_index <= first_index {
            debug!(
                "compact index <= first index, no need to compact";
                "region_id" => self.region_id(),
                "peer_id" => self.id(),
                "compact_index" => compact_index,
                "first_index" => first_index,
            );
            return Ok((resp, ApplyResult::None));
        }
        if self.is_merging {
            info!(
                "in merging mode, skip compact";
                "region_id" => self.region_id(),
                "peer_id" => self.id(),
                "compact_index" => compact_index
            );
            return Ok((resp, ApplyResult::None));
        }
...

and

 else if compact_index > state.get_applied_index() {
        return Err(box_err!(
            "compact index {} > applied index {}",
            compact_index,
            state.get_applied_index()
        ));
    }

We can detect them in advance, and avoid them in advance, by returning false in pre_exec.

Among the 3 cases above, case 1 is more common. Consider the following sequence

apply snap -> compact log -> apply snap

The second apply snap may happen before exec compact log.
After the second apply snap, the truncated index can be updated to greater than the first compact log.

Also consider the following sequence

propose compact log 1 -> propose compact log 2 -> exec compact log 1 -> exec compact log 2(compact index <= first index)

Start by reading the proxy's pre_exec and exec_compact_log paths, then trace the apply-snapshot and successive compact-log sequences described in the issue. Identify how the three failure cases can be detected before execution. Done means pre_exec returns false for compact logs that are already doomed to fail, without performing the unnecessary flush.