add `digest` attribute to `information_schema.processlist` table

[zz-jason]

Feature Request

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

At present, users can kill a connection that executes an expensive query via the kill command. But there might be other connections executing that same expensive query as well. To find these connections, we have to do a pattern match on the query SQL, which is not accurate. A better way is to find all the connections by the query digest.

Describe the feature you'd like:

Adding a digest attribute to the information_schema.processlist table, then we can find all the connections executing an expensive query via the following query:

select id
from information_schema.processlist
where digest = ?; -- fill the digest of that expensive query

Then kill all the IDs in the result.

Describe alternatives you've considered:

N/A

Teachability, Documentation, Adoption, Migration Strategy:

N/A