Use database transactions for builds

[jyn514]

This would have avoided the panic found in #1035.

[syphar]

I'm not sure this is a good idea anymore,

seeing #1011 I would imagine that we record intermediate build steps in the database, which wouldn't work with packing all into one transaction.

I'll close this issue, we can totally reopen when we need this, or if I misunderstood #1035 .