store/tikv: set ScanDetail to true

[coocood]

What problem does this PR solve?

Improve the SLOW_QUERY log, make sure it includes all scan details, so the sum will be accurate.

What is changed and how it works?

Currently ScanDetail is returned only when the scan takes more than one second, set this field to true will make sure the detail always returned.

Before the change, total_keys and processed_keys is missing.

2018/08/21 09:59:11.817 adapter.go:363: [warning] [SLOW_QUERY] cost_time:1.688710205s 
process_time:5.95s wait_time:6.312s request_count:13 succ:true con:1 user:root@127.0.0.1 
txn_start_ts:402343002133495809 database:test table_ids:[31],
sql:select count(c) from sbtest1

After the change, total_keys and processed_keys is present.

2018/08/21 10:00:16.495 adapter.go:363: [warning] [SLOW_QUERY] cost_time:1.397274785s 
process_time:5.278s wait_time:6.254s request_count:13 total_keys:3000187 
processed_keys:3000000 succ:true con:1 user:root@127.0.0.1 
txn_start_ts:402343019173117957 database:test table_ids:[31],
sql:select count(c) from sbtest1

Check List

Tests

• Manual test (add detailed scripts or steps below)

1. start a single instance pd/tikv/tidb.
2. import 3 million records.
3. query select count(c) from sbtest1.
4. observe the SLOW_QUERY log.

Side effects

• Possible performance regression
  TiKV need to allocate memory to marshal the scan detail information in the response. but the impact is very small.

[coocood]

@zz-jason @shenli PTAL

[zz-jason]

LGTM

[ngaut]

/run-all-tests

[zimulala]

LGTM

[shenli]

Should we evaluate the performance regress with sysbench?