Format SHOW PROCESSLIST progress as a tree

[carlosms]

This is a sugestion by @ajnavarro, related to #855.

With the changes in this PR the output of SHOW PROCESSLIST shows the progress formatted as a tree:

mysql> show processlist\G
*************************** 1. row ***************************
     Id: 2
   User: root
   Host: 127.0.0.1:53116
     db: gitbase
Command: query
   Time: 0
  State: running
   Info: show processlist
*************************** 2. row ***************************
     Id: 1
   User: root
   Host: 127.0.0.1:53116
     db: gitbase
Command: query
   Time: 50
  State: 
SquashedTable(repositories, commits) (0/3 partitions)
 ├─ cangallo (101/? rows)
 ├─ octoprint-tft (101/? rows)
 └─ upsilon (101/? rows)

   Info: select * from repositories natural join commits
2 rows in set (0.01 sec)

[golangcibot]

Error return value of printer.WriteNode is not checked (from errcheck)

[erizocosmico]

you should _ = printer.WriteNode

[golangcibot]

Error return value of printer.WriteChildren is not checked (from errcheck)

[erizocosmico]

Other than the minor checks golangci bot is mentioning, LGTM.

[carlosms]

GolangCI changes made and squashed