Support triggers [community contribution welcome]

Triggers can be supported by gh-ost. This is not on our immediate roadmap, and so it's an ideal opportunity for community contribution.

Some notes:

• as far as I can see, all sorts of triggers could be supported (before|after, insert|delete|update)
• throughout the migration process, nothing is really done; the ghost table should NOT have triggers.
• during the cut-over phase we can copy+paste the triggers from original table to ghost table
• Preferably the triggers are read at migration start, not at cut-over phase
• triggers can be applied on the ghost table after the backlog is drained and the tables are in sync (after this.waitForEventsUpToLock())
• triggers must be removed in any case of rollback, and in such case, before gh-ost resumes writes onto the ghost tables (no problem if original table resumes writes)
• Note that at this time gh-ost supports two types of cut-over, and both must be supported.

Anyone who is interested to jump on this please let me know beforehand. Thank you!

Leggi la logica di cut-over in go/logic/migrator.go, quindi traccia l’avvio della migrazione, entrambe le modalità di cut-over, waitForEventsUpToLock() e la gestione del rollback. Il lavoro è completato quando i trigger vengono letti all’avvio della migrazione, applicati alla tabella _ghost solo dopo che il backlog è stato smaltito e le tabelle sono sincronizzate, e rimossi prima che le scritture riprendano lì durante il rollback.