executor: fix slow_query bug when query with time e... (#16768)

[sre-bot]

cherry-pick #16768 to release-4.0

---

What problem does this PR solve?

The #16445 PR not work, since the precision of slow log is nanosecond, but TiDB time precision is microsecond.

Fix below bug.

mysql>select time from cluster_slow_query limit 1
+----------------------------+
| time                       |
+----------------------------+
| 2020-03-13 10:36:27.299064 |
+----------------------------+
1 row in set
Time: 0.020s
mysql>select time from cluster_slow_query where time='2019-12-30 22:55:47.200716' limit 1;
+------+
| time |  -- should has at 1 item.
+------+
0 rows in set

tidb-slow.log

# Time: 2020-03-13T10:36:27.299063744+08:00
# Txn_start_ts: 415253656258478098
# User: [email protected]
# Conn_ID: 4311306
# Query_time: 0.22649397
# Process_time: 0.015 Wait_time: 0.015 Backoff_time: 0.21 Request_count: 14 Total_keys: 439 Process_keys: 426
# DB: seewo_performance_tidb
# Index_ids: [2]
# Is_internal: false
# Digest: 9ee332c0e08d5c27a3f061aaca1fe1c1d1c016713fc7204ced3430771c88fb33

What is changed and how it works?

Related changes

• Need to cherry-pick to the release branch

Check List

Tests

• Unit test

Side effects

• Performance regression
  • Consumes more CPU
  • Consumes more MEM

Release note

[sre-bot]

/run-all-tests

[breezewish]

LGTM

[breezewish]

/run-integration-copr-test

[Deardrops]

LGTM, wait for #16643

[crazycs520]

/run-all-tests

[crazycs520]

/run-integration-copr-test

[zz-jason]

LGTM