support a new administrative command 'show @@processlist'

[simplelogic666]

Easy to view the correspondence between front and backend session
add a new  administrative command 'show @@processlist'

Included column info:

• Front_Id: as dble show @@connection 'id';
• Datanode:as dble show @@DataNode 'name'
• BconnID: as mysql processlist 'id'
• User: as mysql processlist 'user'
• Front_Host: as dble show @@connection ‘host’;
• db: as mysql processlist 'db'
• Command: as mysql processlist 'Command'
• Time: as mysql processlist 'Time'
• State:as mysql processlist 'State'
• Info: as mysql processlist 'Info'

note: if a front session have no active banckend ,the banckend column info will be show NULL

[PanternBao]

doc updated: https://github.com/actiontech/dble-docs-cn/blob/develop/2.Function/2.1_manager_cmd/2.1.03_show.md

[FlyingMao]

verified version： 5.6.29-dble-9.9.9.9-257432cb7dbab3a91038822a2f86375060a16102-20190225073921