SLOW_QUERY support query slow log at any time.

[crazycs520]

Feature Request

Is your feature request related to a problem? Please describe:

Currently, SLOW_QUERY only support to query data that parse from current slow-log file, can't query data from the rotated slow-log file.

Describe the feature you'd like:

To make SLOW_QUERY support query slow log at any time, need to do:

• Support parse all slow-log file, contain the current and rotated slow-log file.
  • Locate which slow-log files meet the user-specified time range, this is to avoid parse all slow-log file.
• Use Next to get a chunk of SLOW_QUERY data to avoid oom.

Describe alternatives you've considered:

compatibility

If user doesn't specified the query time range, only parse the current slow-log file, this behaviour is the same as before.

Teachability, Documentation, Adoption, Migration Strategy:

The development of this feature can be divided into the following steps:

1. Use Next to get a chunk of SLOW_QUERY data to avoid oom. Move dataForSlowLog function to executor pkg.
2. Make SLOW_QUERY support query slow log at any time.
3. Make CLUSTER_SLOW_QUERY support query slow log at any time.