Refactor plugin management:
- support multiple plugins per executable
- support restarting a plugin process in the event it terminates
- simplify plugin lifecycle management by using separate managers for
each scope (server vs backup vs restore)
Signed-off-by: Andy Goldstein <andy.goldstein@gmail.com>
Handle the case where a BackupItemAction may return nil for updatedItem,
meaning "no modifications to the item". The backupPVAction does this,
and we were panicking instead of accepting it.
Signed-off-by: Andy Goldstein <andy.goldstein@gmail.com>