Move db functions to ClusterTx

[monstermunchkin]

This moves db functions from Cluster to ClusterTx.

Progress:

• internal/server/db/backup.go
• internal/server/db/images.go
• internal/server/db/instances.go
• internal/server/db/network_acls.go
• internal/server/db/network_forwards.go
• internal/server/db/network_load_balancers.go
• internal/server/db/network_peers.go
• internal/server/db/network_zones.go
• internal/server/db/networks.go
• internal/server/db/profiles.go
• internal/server/db/snapshots.go
• internal/server/db/storage_buckets.go
• internal/server/db/storage_pools.go
• internal/server/db/storage_volume_snapshots.go
• internal/server/db/storage_volumes.go
• internal/server/db/warnings.go

[monstermunchkin]

@stgraber ready for review.

[stgraber]

Thanks!