Support many-to-many relationship between local pools and backup pools

[mfinifter]

Configuration should be able to specify a many-to-many relationship between local pools and backup pools. I may want to backup several local pools to the same backup pool, or a single local pool to multiple backup pools (e.g., for rotating off-site backups). Or multiple local pools to multiple backup pools.

Expected behavior is that a backup should happen for all possible combinations specified. For example, suppose I want both salmon_tank and tuna_tank to backup to both backup_ocean and backup_lake. The following backups should occur:

• salmon_tank to backup_ocean
• tuna_tank to backup_ocean
• salmon_tank to backup_lake
• tuna_tank to backup_lake

In the common case, only one of the backup pools will be available. This is fine; fail silently (but generate a log message) for any backup pool that isn't available.

[mfinifter]

Closing because #8 will resolve this.