pt-slave-restart doesn't work with multi source replication in 5.7

[Francisco Bordenave]

When you setup pt-slave-restart in a multi sourced slave it doesn't work due several reasons:
1- It only checks first channel reported in SHOW SLAVE STATUS output so if failing channel is the second one it will never notice.
2- In a multi sourced slave whenever you set SQL_SLAVE_SKIP_COUNTER >1 you are forced to run START SLAVE FOR CHANNEL 'channel_name' so you can't start it globally with just "START SLAVE"

Ideally we should be able to support monitoring either single or multi sourced slaves (i.e. adding a channel variable) by supporting FOR CHANNEL syntax.

Possible scenarios:
1- Single channel (or single master) slave. Syntax is the same as always:
SHOW SLAVE STATUS
START/STOP SLAVE

2- Multiple channels: accept --channel flag where we can define which channel to monitor. Syntax needs to support:
SHOW SLAVE STATUS FOR CHANNEL 'channel_name'
START/STOP SLAVE FOR CHANNEL 'channel_name'

[Sveta Smirnova (svetasmirnova)]

Thank you for the reasonable feature request.

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PT-712